THE Queen has said she hopes Scots will "think very carefully" about the referendum in a highly unusual contribution to the debate as the two sides prepare to enter the final 72 hours of the campaign neck and neck.

She made the comment as she greeted well-wishers outside Crathie Kirk, near Balmoral Castle.

It followed pleas by senior MPs for her to speak out in favour of the Union, though Buckingham Palace insisted she remained neutral in the debate.

In other developments as the curtain came down on the biggest weekend of political campaigning Scotland has ever seen:

l More than 1,000 Yes supporters demonstrated outside BBC Scotland's Glasgow HQ in protest at allegedly biased coverage of the referendum;

l Former Prime Minister Gordon Brown revealed MPs will debate further powers for Holyrood on October 16, in the event of a No vote;

l A flurry of polls suggested the No campaign was narrowly in the lead, with the latest poll of polls, averaging out the most recent surveys, putting the lead at 51 per cent to 49 per cent;

l Alex Salmond told supporters Thursday's vote could be their last chance for a generation to secure independence;

l John Reid, the former defence secretary, accused Mr Salmond of using anti-English sentiment to win backing for independence.

Tens of thousands of activists took to the streets over the weekend to galvanise support and try to win over wavering voters.

Today Prime Minister David Cameron will campaign in Scotland for the last time before polls open, while Deputy First Minister Nicola Sturgeon will lead an effort to persuade pensioners, the age group least likely to support independence, to vote Yes.

The Queen's comment came as she greeted well-wishers outside Crathie Kirk, where she is staying with other members of the Royal Family.

She spoke to a group of three or four Scots and a handful of English visitors in the crowd.

Responding to a comment from a well-wisher, she was reported to have said: "You have an important vote on Thursday.

"I hope everybody thinks very carefully about the referendum this week".

Unusually for an ordinary Sunday service, photographers were invited to capture the Royal party leaving the kirk, and witnessed the exchange.

Buckingham Palace declined to comment on a private conversation and neither side in the campaign tried to capitalise on the remarks.

It came after the Queen had talks with Prime Minister David Cameron at Balmoral last weekend.

There were also separate calls from senior Conservative and Labour MPs for her to speak out in support of the Union, as she did in 1977 amid growing clamour for devolution of power to Scotland and Wales.

She used a Silver Jubilee speech to say: "I cannot forget that I was crowned Queen of the United Kingdom of Great Britain and of Northern Ireland.

"Perhaps this Jubilee is a 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."

Royal aides have stressed the Queen's neutrality in the debate, believing independence is a matter for the Scottish people.

The Queen, who remains above the political fray as a constitutional monarch, observed the proprieties of not endorsing either side in the referendum.

A Buckingham Palace spokeswoman said: "We never comment on private exchanges or conversations. We just reiterate what the Queen has always said: she maintains her constitutional impartiality. As the Queen has always said, this is a matter for the people of Scotland."

Last week Mr Salmond said the Queen would be proud to be monarch of an independent Scotland.

Last night a Yes Scotland spokesman said: "Her Majesty is echoing the message from Yes Scotland to all voters - to think very carefully about this one opportunity that Scotland will have on Thursday to choose our future. Of course, Buckingham Palace has been at pains to stress that the Queen has no position on the independence referendum."

A Better Together source said: "The Queen has been very clear that she doesn't want to comment on the referendum."

Yes supporters, meanwhile, vented anger at the BBC outside its Scottish base, claiming its coverage was biased towards Scotland staying in the UK.

Protesters waving saltires and banners called for the BBC's political editor, Nick Robinson, to be sacked. The protest followed a clash between Mr Salmond and Mr Robinson at a press conference last week and a claim by the First Minister in yesterday's Sunday Herald that "of course" the BBC was biased.

A report out today from the centre-right Centre of Policy Studies claims the SNP has seriously underestimated the economic risk of leaving the UK, arguing there could be a £13.8 billion black hole in Scotland's finances in 2015/16, the year Mr Salmond aims to declare independence.

The report said the Scottish Government had overestimated oil revenues and underestimated the impact of financial institutions leaving Scotland and the cost of public-sector pensions. A spokesman for Finance Secretary John Swinney said the study was "full of basic factual errors".