The Queen is said to be privately horrified at the prospect of Scotland voting in favour of independence from the UK.
It is believed the referendum – which could make the monarch the last Queen of Scotland – dominated her discussions with Prime Minister David Cameron on his annual visit to Balmoral at the weekend.
The growing panic over the break-up of the UK was fuelled by a YouGov poll which put the nationalist Yes campaign ahead by 51% to 49%.
Buckingham Palace aides said the Queen had asked for daily updates on the state of the campaigns.
A source said yesterday: “The Queen is a unionist, there is now a great deal of concern.
“If there is a Yes vote that puts us into uncharted territory constitutionally . Nothing is certain and her being Queen of Scotland is not a given.”
YouGov found the nationalists had clawed back a 22-point lead held by the Better Together campaign at the start of August.
Nationalist leader Alex Salmond’s private polling reportedly put his campaign ahead 54% to 46%.
In London, George Osborne tried to win over voters flirting with independence by promising to publish within days a cross-party plan to offer more powers to Edinburgh if Scotland stays in the UK.
This would including handing more tax-raising powers and control over welfare to the Scottish Parliament.
But YouGov found just 54% of Scottish voters wanted the Queen as head of state with 31% preferring an elected figurehead.
Across the UK as a whole, just 45% believed she should remain the Scottish head of state if the country votes to break away in the historic vote on October 17.
A senior source on the Yes campaign yesterday said “questions were being asked” over the Queen’s apparent bias after she was reportedly disturbed by the prospect of Scotland breaking away from the UK.
He said: “It’s very seldom you get any personal opinion from the monarch, that’s why we’re dubious.
“It’s important to realise that retaining the Queen as our monarch after a yes vote is in the white paper.
“If we vote yes, that’s the situation that will sustain.
“But we won’t actually become independent until 2016 when the first elections will be to the independent parliament.”
Despite the official line, there have been strong republican voices in the Yes campaign.
Among them was chairman Dennis Canavan.
He said when Prince George was born last year: “In an independent Scotland, the people of Scotland should be given an early opportunity to decide whether they want to retain the monarchy or choose an elected head of state.
“Personally, I favour the latter option, because a hereditary head of state is an affront to democracy and a complete anachronism in a modern 21st century democracy.”
Palace aides were yesterday trying to dampen down any suggestions the Queen had a bias in the vote, emphasising that she remained “strictly neutral”.
A spokeswoman said: “ The referendum is a matter for the people of Scotland – the Queen remains strictly neutral on this, as she does on all political issues.
“The Queen has maintained a close interest in the referendum as she does with all major matters of public debate and is being kept informed by her ministers and officials in the usual way.” Her Majesty, who watched the Highland Games in Braemar, Aberdeenshire, over the weekend has previously stated her praise for the union.
In a speech to MPs on her Silver Jubilee in 1977, she said: “I was crowned Queen of the United Kingdom of Great Britain and Northern Ireland.
“Perhaps the jubilee is the time to remind ourselves of the benefits which union has conferred, at home and in our international dealings, on the inhabitants of all parts of the United Kingdom.”
With the referendum on a knife edge, there were fears Westminster could be plunged into crisis if Scotland split away. Some Tory MPs warned Mr Cameron would have to resign if he became known as the Prime Minister to end 300 years of the union.
There were also suggestions Ed Miliband would come under pressure because the loss of Scottish Labour MPs would damage his party’s chances of forming a government.
The doomsday scenario prompted warnings markets would panic, sending the pound tumbling and damaging Britain’s recovery.
There were also fears the Treasury would be forced to take on Scotland’s debts, adding to Mr Osborne’s borrowing problems. The Chancellor yesterday repeated his warning that an independent Scotland would not be able to share a currency with the UK.
Mr Osborne said: “No ifs, no buts. We will not share the pound if Scotland separates from the rest of the UK,”
He also announced the Tories, Labour and Lib Dems would set out in the next few days a blueprint for handing more devolution to Scotland – known as Devo Max – if the country votes No.
The Chancellor told the BBC’s Andrew Marr show: “You will see in the next few days a plan of action to give more powers to Scotland. More tax powers, more spending powers, more plans for powers over the welfare state. The timetable for delivering that will be put into effect the moment there is a no vote.”
He added: “The clock will be ticking for delivering those powers, and Scotland will have the best of both worlds.
“They will both avoid the risks of separation but have more control over their own destiny, which is where I think many Scots want to be.”
Mr Salmond branded the new proposal a “bribe”. He said: “Are we expected to believe, after hundreds of thousands have already voted, that there’s a radical new deal?”
He added: “This is a panicky measure made because the ‘Yes’ side is winning on the ground.
“They’re trying to bribe us, but it won’t work as they have no credibility left.”
Blair Jenkins, chief executive of Yes Scotland, added: “This is nothing more than a panic reaction by George Osborne to the success of the Yes campaign on the ground and in the polls.”
Better Together leader Alistair Darling described the latest poll as a “wake-up call” and warned the battle would “go down to the wire”.
