Scotland will get more powers after a No vote, David Cameron, Ed Miliband and Nick Clegg will say today as they try to head off one of the Yes campaign's key arguments before tonight's television debate.

The Prime Minister, Deputy Prime Minister and leader of the opposition have signed up to a joint declaration made earlier this year by Scottish Labour leader Johann Lamont, Scottish Tory leader Ruth Davidson and Scottish Liberal Democrat leader Willie Rennie.

The document pledges to "strengthen further" the powers of the Scottish Parliament, in particular in "the areas of fiscal responsibility and social security".

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The declaration is seen as crucial to the strategy of the No campaign. Opinion polls suggest many voters are keen to see Scotland remain part of the UK, but with extra powers for Holyrood.

But Yes campaigners are hoping the debate will prove a "game changer" after another poll at the weekend showed little change in support for either side.

Pollsters Survation put Yes on 40 per cent, down one point compared with the previous month, with support for a No vote unchanged on 46 per cent.

The Scottish Government accused the parties of reheating the announcement ahead of tonight's head-to-head debate between Alex Salmond and Alistair Darling.

Mr Miliband has said his party would pledge to bring in a new Scotland Bill in his first Queen's Speech if he is elected Prime Minister in next year's General Election.

But today's declaration contains no new specifics about what the other leaders would promise.

It reads: "The Scottish Labour Party, the Scottish Conservative and Unionist Party and the Scottish Liberal Democrats have each produced our own visions of the new powers which the Scottish Parliament needs.

"We shall put those visions before the Scottish people at the next General Election and all three parties guarantee to start delivering more powers for the Scottish Parliament as swiftly as possible in 2015.

"This commitment will deliver a stronger Scottish Parliament in a stronger United Kingdom."

Ms Lamont said: "The ambition for most Scots is a strong Scottish Parliament with more powers but backed by the security and stability of the United Kingdom - this can be delivered with a No vote on September 18."

Ms Davidson insisted saying "No Thanks" in the independence debate would not mean no change.

"It means we can get on with building a more responsible and a more powerful Scottish Parliament while remaining part of the UK family of nations."

For his part, Mr Rennie said the declaration showed all of the main UK party leaders were committed to further devolution of powers to Scotland. "People can be confident that more powers are guaranteed," he said.

A spokesman for the First Minister called the promises included in the declaration "vague". He said: "No one in Scotland will be fooled by this Westminster-led rehash of vague promises and unspecified …powers in the event of a No vote - the Tories have tried that before.

"David Cameron fought tooth and nail to keep a more-powers option off the ballot paper, so how can anyone take him seriously now?"

He added: "The reality is that only a Yes vote on September 18 will give Scotland the powers we need to create a more prosperous and fairer society."

lDavid Cameron will watch tonight's independence TV debate - after Downing Street initially suggested he would miss the highly-anticipated showdown because he is on holiday.

The Prime Minister is due to resume his family break in Portugal later today.

Yesterday No 10 indicated that he would not be tuning in to watch Alex Salmond and Alistair Darling debate.

The Prime Minister's official spokeswomen explained away the snub, saying: "I'm not sure that Scottish TV debates will be shown on television in Portugal".

But STV, which is broadcasting the first of the TV debates, said that the event would "be available internationally" over the internet.

Pete Wishart, the SNP MP, accused Mr Cameron of being afraid to watch.