Polling has closed in Scotland's independence referendum, and the counting of ballots has started, with the result expected in the early hours of tomorrow.
With 97% of the electorate registered to vote, officials expect a turnout as high as 80%.
Support for Scottish independence amounts to 46% of the electorate with 54% wanting to stay in the United Kingdom, according to a poll by YouGov carried out on the day of the referendum.
YouGov contacted Scots it had previously canvassed in recent days and its survey is not akin to a full exit poll.
Just under two weeks ago, it produced a poll that briefly put the independence campaign ahead, sending shockwaves through the British establishment.
YouGov research manager Laurence Janta-Lipinski said the organisation's last poll had picked up a "small but significant late swing from Yes to No on polling day".
"This YouGov poll indicates the union has prevailed – it certainly looks more comfortable than it did 10 days ago," Mr Janta-Lipinski told Reuters. "It look like the union will remain intact for the time being."
YouGov president Peter Kellner said he was 99% certain that Scotland would vote to remain in the UK.
He told Sky News: "At the obvious risk of looking like a complete prat in eight hours' time, I would say it is a 99% certainty of a No victory... I can't see No losing this now."
Mr Kellner said today's research had suggested "substantially more" people switched from Yes to No than the other way around.
He added: "'If we have got this wrong, if Yes win, we and by extension other pollsters, have got something badly wrong."
The chairman of Yes Scotland said he was not conceding defeat, despite the YouGov survey.
Former Labour MP Dennis Canavan told Sky News: "I'm still optimistic ... I'm not at this stage conceding the result."
Mr Canavan said it was "probably correct" that the vote would settle the independence question for a lifetime.
He said that while the Yes camp had fought a "very positive campaign, a magnificent campaign", the No message was characterised by "a bit of negative scaremongering going on, a bit of collaboration, perhaps even collusion, on the part of the British establishment".
Scottish referendum ballot explained
The referendum came more than three years after Alex Salmond's SNP secured a landslide victory at Holyrood.
The polls closed at 10pm and the first results should be known by early tomorrow morning.
There is no exit poll for the vote. The crucial ballot, which could see the 307-year-old union between Scotland and England brought to an end, was expected to go down to the wire.
After a frenetic final day of campaigning from the rival sides yesterday, the turnout is expected to have been high, with 4,285,323 people registered to vote, according to the Electoral Commission.
For the first time 16 and 17-year-olds across the country were able to take part.
The question voters faced was a simple one: "Should Scotland be an independent country?"
With the momentum in the final weeks of the campaign appearing to be behind the Yes campaign, the leaders of the three main Westminster parties all pledged to give Scotland more powers if the outcome is No.
But nationalists dismissed this, insisting only a Yes vote would give Scotland the powers it needed.
RTÉ News will bring the latest on the Scottish referendum through the night on RTÉ.ie/news
Close to the famous rugby ground, I'm at Murrayfield Parish Church polling station for @morningireland #indyref pic.twitter.com/wprN1Dbqu2
— Paul Cunningham (@RTENewsPaulC) September 18, 2014