He is a man of supreme wealth, one of Britain's greatest entrepreneurs.

Yet Mike Ashley remains distant, elusive, a man of mystery who shuns interviews. For all his success and visibility, few people know or understand him. This is the way the Sports Direct tycoon wishes it to be. His mystique has now transported itself north of the border, given his recent interest in Rangers. Ashley has taken a 9% stake in the Ibrox club, and has already removed key figures from the Rangers boardroom, such as CEO Graham Wallace, and installed his own men in their stead. Remarkably, given his relatively minor stake, Ashley now exerts significant control at Rangers, in a saga that is set to run and run.

He has been called "a retail genius", building his massive Sports Direct empire to the tune of 400 UK stores and 24,000 employees, all the while remaining out of reach to the public. As owner of Newcastle United, Ashley went five years from 2009 without uttering a single comment in public about the club. Instead, he brought in trusted confidants - such as Derek Llambias, whom he has now installed at Rangers - to do his bidding. Ashley is no recluse; he has just come to enjoy telling everyone next to nothing.

In recent weeks I have spent days poking around the fringes of the Ashley empire, from his old school in Buckinghamshire to his current Sports Direct HQ at Shirebrook in Nottinghamshire, all along being met with either morsels of comment or a stony silence. Even Ashley's various PR people go shtum when you raise innocent queries about him.

"Mike Ashley detests speaking in public," one city analyst told me. "For all that his business empire is huge - and successful - he sees no need to make himself a public commodity. You will never, ever hear Ashley on Desert Island Discs."

The mystery is, why has he got involved in Rangers? What is to be gained from it? The size of the Glasgow club, and its vast tribe of supporters with their retail value, has been cited as one reason, and there is surely something in this. Yet Sports Direct is valued at £4bn and last year boasted a rampant turnover of £2.7bn, well up on previous years. "Rangers' value is a pebble in the ocean for a guy like Ashley," one observer told me.

A more likely answer to the "why Rangers?" question around Ashley lies in one word: Europe. Sports Direct, having spent over 25 years aggressively building up its dominance in Britain, now has a distinct strategy about spreading out across Europe, and has 120 stores in various countries including Belgium, Austria, Germany and the Baltic nations.

"We have European expansions plans," said Dave Forsey, a long-time associate of Ashley, who is Sports Direct's CEO. If Rangers FC is ever restored to the game's high table - as it surely will be one day - then the club will once more be in high-visibility European competition, with Sports Direct's brash blue and red livery clad all over it. Ashley, not for a first time, has seen a branding opportunity.

Ashley is widely known to enjoy a pint of beer - mainly lager - and at one Newcastle game a TV camera, which happened to focus on him, filmed him downing what looked like at least half a pint in one go, in a few seconds. It is hard to believe now, when you see the famous Ashley girth, but he was once a champion squash player who might have gone on to greater things in that sport had injury not set him back. That misfortune back in the late 1970s, which riled Ashley at the time, came to be the trigger for the great achievements in his life.

He has a modest background - his father was a manager in a food distribution depot - and he was a determined but average pupil at Burnham Grammar School in Buckinghamshire, a school which to this day prides itself on community values and self-help among its students.

A rare comment about Ashley from back then came from Margaret Fleet, now retired and living in Bath, who during Ashley's time was a depute-head at the school.

"Most kids are fairly malleable, and you can subtly change their views about things, but Mike Ashley was different," Fleet said. "I remember him having strong views about things and being quite determined, and it doesn't surprise me that he has gone on to be successful. I remember him talking to friends about his Saturday job in a sportswear shop, and talking about how one day he would own the shop."

From that shop in Maidenhead in 1981, where the teenage Ashley first got a whiff of the sports retail business, a corporate giant duly emerged. Ashley scrubbed together enough finance to open his own shop in the town and then discovered he had the wits to expand and buy-out rivals and buy up sports franchises to further enlarge his empire. Over these 30 years a shop he first christened Sports Direct back in 1982 has become both unmissable and unmistakeable in almost every British town, and now, increasingly, across Europe.

Ashley's lifestyle with it became lavish. His divorce to Linda Jerlmyr, a Swede, in 2002 cost him a reported £50m settlement. Whether that figure is accurate is, like so many things connected to Ashley, shrouded in mystery. His most recent reported residence is said to be in Totteridge Lane in London, the "millionaires row" where some residences can set you back a cool £5m. One newspaper report stated that his new house has 33 rooms. Another said it had 15 rooms. Whatever, it's a far cry from his modest upbringing in a bungalow.

Ashley now travels by private helicopter. Most weeks he flies north to Shirebrook, the former pit village which is the unlikely setting for Sports Direct's HQ, where Ashley oversees operations and meets with his inner circle, such as Dave Forsey.

Ashley's sometimes shambling appearance is deceptive. He often wears scuffed jeans, with his shirt either rumpled or hanging out, and has been seen to carry his mobile phone and wallet in a plastic bag. But this is as far as his man in the street image goes.

Ashley's business figures became astounding. He made £925m for himself on the day Sports Direct was floated in 2007. He maintains a 60% stake and his current wealth has been valued at £3bn-plus. Last year Sports Direct's rampaging success continued unabated, with pre-tax profits up 15% to £240m on total sales worth £2.7bn, itself a 24% leap. Is there anything Mike Ashley cannot do successfully?

Well, yes there is, according to Newcastle United fans, whom Rangers supporters are now listening to very attentively. It is fair to say, having shelled out over £250m of his wealth on the English Premier League club for ownership in 2007, Ashley's experience on Tyneside has not been without unhappiness. A section of the Toon Army detest him, contending that Ashley's interests are all wrapped up in Sports Direct, with next to no commitment to the football club.

"Ashley has no real interest in football at all," says Graeme Cansdale, a Newcastle United fan who is campaigning to get him out of the club. "He is a corporate vulture, he has leached off our football club. It is very, very sad what has happened to Newcastle United under Mike Ashley.

"In sport, and in football, you want to do your best, you want to fulfil your potential. But, in terms of Newcastle United, Ashley isn't the slightest bit interested in that.

"His sole interest is his business. Ashley uses St James' Park to get free advertising for his company all over the stadium. That could be money that went into the football club. He just shows complete disinterest in the football side.

"Ashley is a huge millstone around the neck of Newcastle United. In 2007 the club's debt was around £80m. Now it's around £129m, if you factor the money the club owes him. Now no-one wants to buy it off him.

"In fact, Newcastle United is no longer a football club under Ashley. It is very, very sad. If Ashley left this club yesterday it would be too late for us."

As Ashley's Newcastle dream has turned sour, at least in terms of his public popularity, these are the sort of stinging comments that are now emerging. Just as he aggressively bought up Slazenger, Dunlop and Donnay for Sports Direct's benefit - nothing wrong in that - so he is viewed as using Newcastle United for his own ends.

"It is shocking what Ashley has done at Newcastle United," says Mark Jensen, another fan who edits TheMag website, a forum for Newcastle fans. "He has robbed the supporters of all hope - the very thing that keeps football supporters going.

"Ashley's view of the football club appears to be, 'spend as little money as possible, and rake in whatever I can.' I actually think, while investing very little in the team, he has raked in tens of millions in terms of the merchandising and free advertising he has secured for himself.

"When he first arrived Ashley was welcomed by the Newcastle fans. Now, seven years on, we realise what it all entails with him. His interest is all the revenue streams he can get his hands on. Like I say, he has robbed Newcastle fans of all hope."

These sentiments from Tyneside have made some Rangers fans wary. What is the truth about Ashley? Will he simply plunder Rangers for the club's retail deals to effectively line his own pockets, or will he, via his enormous wealth, restore the club to its old glories? Or might he do both, in which case it might be a case of better the devil you know?

As ruthlessly as Ashley is depicted, he has had his setbacks, and not just at Newcastle United. In 2007 Sports Direct, following its stock market flotation, had an extremely rocky time, when its shares plunged from 300p at launch to just 30p. Ashley was said to be very depressed about it all. One onlooker said he was "badly beaten up by the City".

In 2008, following a row with Kevin Keegan, then the Newcastle manager, Ashley announced that he would be selling the club, and that he had made a mistake in buying it. In October 2009 Keegan won a constructive dismissal case against Newcastle, and Ashley was forced to make a £2m pay-out to him.

In 2009, in a rare outburst from him about Newcastle United, Ashley again admitted that he had erred in buying the club. His sentiments also appeared to clash with those of his opponents who were claiming he "leeched" off Newcastle United.

"I have to put £20m a year into the club," said Ashley. "I spend more than every fan put together puts into the club each year. I have never said I was an expert in football clubs. Of course I regret it [buying Newcastle United]. I tried my best but my best was woefully short."

That was in 2009. Five years on, and Ashley is still the Newcastle United owner, with his clutches now being felt at Rangers. He has said he won't sell Newcastle until 2015 at the earliest, and in those intervening six years Ashley's grip - or the grip of those he employs to look after his football interests - has been felt harshly on Tyneside.

At Newcastle a total of six newspapers - the Daily Telegraph, the Express, the Independent, the Sunday Sun, the Journal and the Chronicle - have all been banned at one time or another from reporting from St James' Park. Whether these edicts came from Ashley himself or his adjutants is beside the point - he appears to have become more sensitive to adverse comments or criticism about his methods.

In and around Sports Direct Ashley has an inner sanctum of figures whom he trusts implicitly. Among these are Forsey, his CEO and long-time friend and colleague; Llambias, who was Newcastle United's managing director and is now installed at Rangers; Keith Hellawell, the former police chief constable of Cleveland who is now Sports Direct's chairman; and Keith Bishop, an ex-journalist turned PR guru who attempts to protect Ashley's reputation. In truth, it is quite an impressive praetorian guard.

It is interesting that, when news emerged of Ashley's involvement at Rangers, some of the club's more militant followers directed their (peaceful) protests at some Sports Direct outlets in Glasgow. Bad publicity was their aim, and maybe they had a hunch about Ashley.

Graeme Cansdale, who has watched the Ashley regime for seven years in Newcastle, claims to have seen a shift in his attitude, in particular towards Sports Direct's reputation.

"Some people think that Ashley is untouchable, but he doesn't appear to be, says Cansdale. "Sports Direct is Mike Ashley's baby, and he seems hugely sensitive about anything that is sad or written about it.

"Last year about 800 Newcastle fans organised an anti-Ashley march. It was basically people expressing their disenchantment with Ashley and Sports Direct's involvement at Newcastle United. The papers covered it, and people talked about it, and it seemed that Ashley - or at least the people Ashley surrounds himself with - got very touchy about it all.

"We've seen some newspapers being banned from St James's Park. We've also seen the fans' Trust being barred from the fans forum at the club.

"Basically the fans were saying that Ashley - and specifically Sports Direct - were bad for our football club. Sports Direct received a lot of negative comments from supporters, and Ashley appeared not to like it one bit. Like I say, Sports Direct is his baby, he has built it up from nothing, and he does not like any negative comment about it."

One reason for this might be that, within Sports Direct itself, Ashley prides himself on his employees working conditions. He has been slated in some quarters for using zero-hour contracts, though against that, Ashley is set to hand out 34 million shares in the company to 3000 of his workers if profit targets are reached in 2015. Staff turnover at Sports Direct is unusually low, implying employee satisfaction, and one factor for it is believed to be Ashley's generous share-bonus schemes.

More than one person has attested to Ashley's groundedness. The tycoon is said to drink with the same mates he had 30 years ago, before fame and wealth washed over him. He attends staff parties, enjoys a pint, and is said to mix well with folk. Around the Shirebrook area there are pubs and Chinese restaurants which Ashley frequents, where he and his colleagues enjoy their fun and their business pow-wows. "It has been said more than once that, for those who know him, Mike is a good egg," one analyst said.

In Glasgow, the test awaits Ashley. Does he want full-on power at Rangers, or is he happy just to be a minor shareholder, protecting his retail deals at the club? And will he court fan-popularity, or not give a stuff, which for years was his policy at Newcastle.

One interesting observation about Ashley is that, when Newcastle or Rangers fans have presented an unflattering image about his activities, he chooses not to respond. Ashley never fights his own case, never offers his own narrative. He is content to preside over the ongoing success story of Sports Direct.

He is one of Britain's most reticent, elusive, enigmatic billionaires. And his new port of call is Ibrox Stadium in Glasgow.