A new opinion poll being published on Sunday put Scotland's pro-independence camp ahead for the first time in the campaign, just 11 days before the referendum on splitting from Britain.
The YouGov poll for the Sunday Times newspaper 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 the Scottish National Party's campaign.
The Sunday Times also reported that the British Queen now feels "a great deal of concern" over the independence vote and has asked for daily updates.
The government of David Cameron is now expected to reveal last-minute concessions - including the devolution of more powers to Scots - in an attempt to keep the 300-year-old union alive.
The Better Together campaign, which backs Scotland staying in the UK, 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. But another YouGov survey for the Times newspaper on Tuesday showed a marked narrowing of the gap to six points.
In response to the latest poll, Alistair Darling, leader of the Better Together campaign, 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."
The chief executive of the "Yes" campaign, Blair Jenkins, urged his side to remain focused on the campaign.
He added of the YouGov survey: "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."