Narrow lead for Remain as Scottish turnout estimated between 70 and 80 per cent

To add further uncertainty to the outcome, 12% of those polled said they could switch sides as they head for the polling booth.

With turnout key to the result, Scotland's Chief Returning Officer, Mary Pitkeithley, told the BBC she estimated that turnout in Scotland could be as high as 70-80 per cent. 

READ MORE: When will the results of teh EU referendum be known?

Ukip leader Nigel Farage insisted his side could pull off an historic victory if "soft" In voters stayed home.

"Actually I do think we are in with a very strong chance, I do genuinely. But it's all about turnout and those soft Remainers staying at home. I do think that the people who have decided to vote Leave have a real conviction and passion," Mr Farage said.

However, early indications showed long queues snaking outside voting centres as the nation braved the storms to decide Britain's future role in Europe.

But some polling stations were forced to close, and many others are flooded due to the heavy rain sweeping the capital.

Kingston upon Thames Council in south west London moved two polling stations after they became inundated with water.

READ MORE: Nicola Sturgeon says she voted to remain in EU with her head and her heart

Prime Minister David Cameron ignored questions about the weather, saying only "Good morning" to the gathered media from across the world, as he and wife Samantha cast their votes at Methodist Hall in Westminster.

Labour leader Jeremy Corbyn arrived in a positive mood to cast his vote in his Islington constituency in north London.

Asked if he was feeing confident, Mr Corbyn smiled and said: "Extremely, it's a very good day."

On the outcome of the referendum, he joked: "You could either check the wind or check the bookies," adding "the bookies usually get it right".

Pro-Leave Justice Secretary Michael Gove said he was feeling "quite excited" as he accompanied his wife, Sarah Vine, to a polling station in North Kensington.

READ MORE: The money markets, bookmakers and betting exchanges point to a Remain vote

The referendum has seen one of the longest, and most personally bitter, campaigns in recent British political memory.

Both sides of the campaign have been locked in fierce fighting for months, and things came to a frenetic close on Wednesday as senior politicians criss-crossed across the country to try to sway undecided voters.

The Prime Minister and his Remain colleagues from across the political spectrum have warned of the potentially severe economic consequences of a Brexit vote amid fears of financial market turmoil and another recession.

READ MORE: All you need to know about voting in the EU referendum, and what happens next

But Leave campaigners, led by Tory heavyweight Boris Johnson, have urged voters to "take back control" of the country.

Mr Farage would not elaborate on his reasons for missing Wednesday night's final Channel 4 debate of the campaign, saying only that it was "for family reasons".

He came in for particularly stern criticism after unveiling a Brexit poster showing a queue of hundreds of immigrants arriving in Europe with the slogan "breaking point".

And Leave campaigners were left furious after Chancellor George Osborne made use of Treasury research to warn that quitting the EU would result in households being £4,300 a year worse off.

Meanwhile, dozens of celebrities have intervened during the course of the campaign to make their feelings known.

Footballer David Beckham, James Bond actor Daniel Craig and Harry Potter author JK Rowling were just three of the high-profile names to back the Remain campaign, while Leave won support from the likes of comedian John Cleese, former cricketer Sir Ian Botham and former England football player Sol Campbell.

A record number of voters are eligible to take part in the referendum, with the Electoral Commission putting the number at 46,499,537.

Polling stations will close at 10pm.

Fieldwork for Ipsos Mori's survey of 1,592 people was carried out until 9pm on Wednesday night.

Vote Leave chief executive Matthew Elliott pleaded for Brexit supporters to vote - warning that turnout was high in Scotland and London, areas expected to back Remain.

In an email message he said: "There is a very real chance that voters in London and Scotland will vote to keep us in the EU today despite the heartlands of the country voting to leave."

The message included a photo of a queue outside a polling station in a "leafy London suburb".

READ MORE: The money markets, bookmakers and betting exchanges point to a Remain vote

Mr Elliott added: "If you don't want people in London to force you and your family to stay in the EU please email and call all your friends and make sure they vote Leave today."

Asked if he was feeling confident ahead of the result as he waited to board a flight to London at Edinburgh Airport, Mr Johnson said: "I think the polls have been very close.

"From what I have heard and all the information is that turnout is good in areas where we need it to be."

Asked if poor weather could affect turnout in key parts of the country, he added: "l don't think that will make any difference in the long run.

"It's a long time to go until the polls close."

Boris Johnson only just made it in time to cast his vote - arriving just 25 minutes before the polls closed.

He had been attending his daughter's graduation in St Andrews earlier in the day, but ran into difficulty when his flight back from Edinburgh was delayed.

The former mayor of London arrived at his polling station in Islingston, north London, with his wife Marina Wheeler, to some heckles from the crowd.

READ MORE: The money markets, bookmakers and betting exchanges point to a Remain vote

But Mr Johnson did not appear stressed, telling the Press Association he was "not at all" worried about not making it in time.

Asked how he felt about the vote he said: "Let's see, let's see. It's in the hands of the people now."

He added that the graduation had been "wonderful", before making the short journey home on foot, accompanied by police officers.