One thing is for certain: Mike Ashley is a phenomenally successful businessman.

The 50-year-old sees an opening, an opportunity, and he plunders it. The financial figures around Ashley back this up emphatically.

Ashley's Sports Direct retail business, with more than 400 stores which someone has described as "looking like chaotic jumble sales", is now valued at more than £4bn. Having floated the company - against nearly everyone's advice - in 2007 Ashley's own 58% stake in Sports Direct is currently worth £2.8bn.

Last year, MASH, the investment vehicle Ashley uses for his swoops on the likes of Newcastle United and Rangers, returned a profit of £256m. Ashley's current private wealth, estimated to be £3.65bn, places him among western Europe's most prolific and creative opportunists. Dave Forsey, Sport Direct's chief executive, has lauded Ashley's "passion, enthusiasm and creativity".

And yet . . . Rangers beware. The tidings are dire from the north-east of England about a man who is believed to be homing in on power at Ibrox.

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

Cansdale is one of a vast throng of Newcastle fans who, having hailed Ashley as a saviour when he bought the club in 2007, now believe they called it drastically wrong. Instead, an owner has emerged who swallowed up the club's merchandising business, turned St James' Park into a shrine to Sports Direct and has left Newcastle mediocre on the field. The side are, currently, in Barclays Premier League peril.

"In sport, and in football, you want to do your best, you want to fulfil your potential," says Cansdale. "But, in terms of Newcastle United, Ashley isn't the slightest bit interested in that. His sole interest is his business. He will consume your brand. He has done it in the sports retail world itself, in terms of the companies he has swallowed up, and he has done it to us at Newcastle United. Our club's merchandising arm has been virtually consumed by Sports Direct.

"Meanwhile, Ashley uses St James' Park to get free advertising for his company all over the stadium. That could be money that went into the club. He just shows complete disinterest in the football side. He's never been concerned at all with the actual club.

"Ashley is a huge millstone around the neck of Newcastle United. In 2007 the club's debt was around £18m. 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."

Cansdale's comments were echoed by many others yesterday. There is a widespread soreness among the Newcastle fans at the way Ashley is seen to have run their club, including his perceived syphoning of revenue streams for the benefit of Sports Direct and a basic lack of football ambition for the club itself.

Ashley paid £134m for the club seven years ago and immediately provided £129m in interest-free loans. None of this, though, endears him today to the Toon Army. "He has robbed the supporters of all hope - you know, the very thing that keeps football supporters going," says Mark Jensen, who edits TheMag website, a forum for Newcastle fans.

"Ashley's view of the 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."

Jensen agrees that many Newcastle fans feel they deluded themselves at first. "When he first arrived Ashley was welcomed by the fans," he says. "Now, seven years on, we realise what it all entails with him. His interest is not the actual club, not at all. His interest is all the revenue streams he can get his hands on. Like I say, he has robbed Newcastle fans of all hope."

Chris McQuillan, another Newcastle fan and activist, who runs the Time 4 Change campaign, believes that Ashley will implement at Rangers exactly what he has done in England's north-east, in terms of the benefits to his own business empire. But with one crucial difference: Rangers, unlike Newcastle, should offer Ashley Champions League football sooner rather than later and a European stage on which to flash his wares.

"I think Ashley will get rid of Newcastle United, and he's a shoo-in for Rangers," says McQuillan. "At Rangers he will surely at some point get Champions League exposure for his business. He can't do that at Newcastle, because getting into the Champions League from the English Premier League costs him too much money. But he will get that at Rangers, even if he runs the club relatively badly."

McQuillan vividly recalls being at St James' Park in 2012 when Newcastle played Atromitos of Greece in a Europa League qualifier.

"All Ashley has done is turn Newcastle United into a massive billboard for his own business empire. It's all about a platform for Sports Direct, nothing else interests him. That night when we played the Greek team I remember Sports Direct logos flashing around our stadium, except with Greek dot.com addresses. That is Mike Ashley. With Sports Direct he has almost taken over the merchandising side of this club on the sly."

Ashley, of course, already has an arm-lock on both Rangers' retail income and the Ibrox naming rights, thanks to Charles Green. Some believe, given Ashley's business zeal and success, he may actually be a success at Rangers. But few in Newcastle believe this.

"I warn Rangers fans . . . be ready for this," says Graeme Cansdale. "Have no illusions about him. Mike Ashley has been terrible for Newcastle United. We are now a club with absolutely no ambition. The spirit has gone."