The three main political parties are poised to make an “unprecedented joint pledge” which would promise more devolved powers for the Scottish Parliament if the No campaign wins out in the September referendum on independence.

The Conservatives, Labour and the Liberal Democrats will meet at the end of this month to reaffirm their plans to give Scotland more powers within the framework of the United Kingdom, the Herald reported.

Details of the agreement emerged as the campaign entered its final 100 days on Monday.

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Alistair Darling, leader of the Better Together movement, is drumming up support for a “future as part of the United Kingdom with substantially enhanced powers for the Scottish Parliament”, as Westminster seeks to prove it is sincere about giving Holyrood more control of its own affairs.

The former Labour Chancellor will tell supporters in Glasgow: “When voters go to the polls on September 18, I want every voter to understand that within the United Kingdom change and progress is coming to Scotland, under-pinned by the commitments of all three parties. We will be offering the guarantee of a constitutional future for Scotland which corresponds with what the great majority of Scots have told us they want.”

Sources in the party told the newspaper that this commitment will be formalised in a joint three-party event in the next two weeks. The Conservatives, Labour and the Lib Dems have reportedly each set out proposals for devolution but are yet to agree on specifics.

A joint statement would reflect these differing views but would also be a general “renewal of vows” on handing over powers to Scotland, one source told the Herald. The three main parties are keen to counter SNP claims that they will abandon their proposals if voters reject independence.

On Sunday, First Minster Alex Salmond said it would be “very foolish to rely on promises from Unionist parties”.

Speaking on the BBC's Andrew Marr Show, the SNP leader said: “We shouldn't rely on pre-referendum promises by parties which are under pressure. The only guarantee of getting more powers is to vote Yes on 18 September.”

Last week, a new poll suggested that the majority of English and Welsh people want Scotland to remain part of the UK.

The Financial Times poll found that 55 per cent of those who expressed a strong preference want Scotland to stay while 15 per cent support a split.

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The other 30 per cent had no strong opinion.

And President Obama also waded into the debate, claiming that the US had a strong interest in the UK remaining a “strong, robust, united and effective partner”, but added the decision was up to the people of Scotland.