A new opinion poll being published Sunday put Scotland's pro-independence camp ahead for the first time in the campaign, just 11 days before a referendum on splitting from Britain, AFP reported.
The YouGov/Sunday Times poll gave the "Yes" camp 51 percent support compared to the "No" camp's 49 percent, excluding undecided voters.
Although the two point lead is within the margin of error, the findings dramatically up the stakes ahead of the vote on September 18, handing valuable momentum to First Minister Alex Salmond's Scottish National Party (SNP).
The Sunday Times also reported that Queen Elizabeth II now feels "a great deal of concern" over the independence vote and has asked for daily updates.
Those fighting to keep the 300-year-old union, who include Prime Minister David Cameron, are now expected to announce key concessions in a bid to fight back.
Any vote for Scotland to leave Britain would be a landmark event raising a string of questions about Britain's standing in the international community.
Scotland represents one-third of Britain's landmass and is home to Britain's Trident nuclear deterrent, which the SNP says must be out of Scotland by 2020 in the event of independence.
The Better Together campaign, which backs Scotland staying in Britain, has been ahead in opinion polls across the board for months but its lead has narrowed in recent days.
The "No" camp had a 22 point lead in YouGov polling just one month ago.
In response to latest poll, Alistair Darling, Better Together's leader, said it showed the referendum "will go down to the wire".
"We relish this battle," he added. "It is not the Battle of Britain -- it is the battle for Scotland, for Scotland's children and grandchildren and the generations to come. This is a battle we will win."
Another YouGov survey for the Times newspaper on Tuesday showed a marked narrowing of the gap, with 47 percent saying they would vote "Yes" and 53 percent "No".
The chief executive of the "Yes" campaign, Blair Jenkins, urged his side to keep focused on the campaign in order to secure victory.
He added: "While this poll puts us marginally ahead, other polls show that we still have more progress to make if we are to win.
"We will be working flat out between now and 18 September to ensure that we achieve a Yes vote."
Salmond has said Queen Elizabeth would remain head of state of an independent Scotland but the Sunday Times quoted an aide as saying this was "not a given".
"The Queen is a unionist," it quoted an unnamed senior royal source as saying. "There is now a great deal of concern."
The British monarch has a home at Balmoral in the Scottish Highlands and on Saturday made her traditional visit to the Highland Games at nearby Braemar to watch pursuits like caber tossing and Scottish dancing.
Cameron is due to visit Balmoral on Sunday, a visit arranged before the poll was published.
He would face pressure to stand down as prime minister from some lawmakers if there was a "Yes" vote, the Sunday Times reported.
He insisted again on Thursday that he would not resign if Scotland split from Britain.
His Conservative Party is unpopular north of the border, where they are often seen as elitist and out of touch with Scottish concerns.
The party has just one lawmaker in the House of Commons out of 59 Scottish seats.
Former British prime minister Gordon Brown, a Scot, blamed Conservative policies for holding back support for the union in a Sunday Mirror article.
The YouGov/Sunday Times poll also showed that pro-union campaigners such as Darling and Cameron were also trusted less than Salmond and his deputy, Nicola Sturgeon.
In response to the narrowing polls, the "No" campaign is reportedly set to announce measures to devolve more powers to Scotland.
The Observer newspaper reported that an announcement was expected within days on plans to let Scots decide on a federalised future for their country after intensive cross-party talks.