The leaders of the three main parties at Westminster have signed a pledge for more powers for Scotland if it rejects independence.

David Cameron, Ed Miliband and Nick Clegg have signed the letter.

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It promises "extensive new powers" for the Scottish Parliament "delivered by the process and to the timetable agreed" by the three parties.

The Yes campaign said a vote for independence in Thursday's referendum is the only way to guarantee Scotland gets the powers it needs.

In the letter, the party leaders say they agree that "the UK exists to ensure opportunity and security for all by sharing our resources equitably across all four nations".

The joint letter also states: "And because of the continuation of the Barnett allocation for resources and the powers of the Scottish Parliament to raise revenue, we can state categorically that the final say on how much is spent on the NHS will be a matter for the Scottish Parliament."

A spokesman for Yes Scotland said: "It's clear that project panic is willing to say anything in the last few days of the campaign to try to halt the Yes momentum - anything except what new powers, if any, they might be willing to offer.

"The reality is that the only way to guarantee Scotland gets all the powers we need to create jobs and protect our NHS is with a Yes vote on Thursday - so that we can use our enormous wealth to create a better and fairer country."

Earlier, former prime minister Gordon Brown said new powers for Scotland in the event of a No vote will unite rather than divide the country.

He said three guarantees must be locked in: to guarantee new powers to the Scottish Parliament, to guarantee fairness to Scotland, and to guarantee the power to spend more on the NHS if that is the wish of Scottish people.

It comes after Mr Brown last week set out his proposed schedule for new powers to be transferred north, saying work on it would start immediately after the referendum.

The Labour, Conservative and Liberal Democrat leaders in Scotland then came together to endorse the timetable for a new transfer of powers from Westminster to Holyrood.

First Minister Alex Salmond described last week's united front by the pro-union parties as a "retreading" and "repackaging" of earlier statements.

He said at the time: "It's totally inadequate, it's not enough. It's nothing approaching the powers that Scotland needs to create jobs, to save the health service and build a better society."

Today, Mr Salmond dismissed the Westminster pledge as a "last minute desperate offer of nothing".

The First Minister insisted the promise of extensive new powers for the Scottish Parliament would not dent the Yes campaign and raised questions about how binding the pledge is.

He told BBC Radio Scotland's Good Morning Scotland: "This so-called vow...I suspect it's been called a vow because the last time one of these leaders made a pledge and signed the pledge was the Deputy Prime Minister Nick Clegg when he signed a pledge that he wouldn't have tuition fees and then promptly put them up to £9,000 for English students in England.

"It's a classic example of how this last minute desperate offer of nothing is not going to dissuade people in Scotland from the huge opportunity of taking Scotland's future into Scotland's hands this coming Thursday."

Labour's Douglas Alexander said the pledge sets out a vision that can unite Scotland.

The shadow foreign secretary told BBC Breakfast: "Here in Scotland, we have been talking about these powers for many months.

"What we're saying today is that we can have the best of both worlds. We can have a stronger Scottish Parliament but with the strength, stability and security of the United Kingdom.

"My sense is that in the closing days, the final hours of this referendum campaign, that vow that we can have faster, safer, better change is actually a vision around which Scotland can unite."

Mr Alexander added: "I think the Yes campaign are struggling. They had an avalanche of facts engulf their assertions last week, when it was announced every major Scottish bank would move their registered office to London.

"We've had a company like Standard Life confirming that it would move operations south. The economic risks suddenly became very real last week.

"At the same time, we're offering what I believe most of us here in Scotland want - faster, safer and better change."