THE three main pro-UK parties are on the brink of an unprecedented joint pledge to guarantee more powers for the Scottish Parliament if voters reject independence.

Labour, the Conservatives and the Liberal Democrats are planning to come together by the end of the month for a renewal of their commitments on devolution in line with the campaign to keep Scotland as part of the UK.

Details of the pact, designed to ­counter claims they are not serious about handing Holyrood more powers, emerged as the campaign enters its final 100 days today.

The milestone will be marked by set-piece events and campaigning across the country.

Better Together leader Alistair Darling will set a 100-days challenge to rally support for a "future as part of the United Kingdom with substantially enhanced powers for the Scottish Parliament".

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."

This is likely to be formalised during a joint Labour, Conservative and LibDem event in the next two weeks, sources in the parties say.

They have each set out proposals to give Holyrood extra powers, including greater control over income tax, though they disagree about the details.

A joint statement would not attempt to reconcile the competing plans, which will be put to voters in the parties' 2015 Westminster election manifestos, but would amount to a general "renewal of vows" on devolution, according to one insider. The aim is to counter SNP claims they will drop their proposals in the event of a No vote.

The charge was repeated yesterday by First Minister Alex Salmond, who said it would be "very foolish to rely on promises from Unionist parties".

Speaking on the BBC's Andrew Marr Show, the SNP leader added: "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."

Yes Scotland claimed it was winning the contest to woo undecided voters. It said private research showed that for every 10 undecided voters, seven were moving to Yes and three to No.

Chief executive Blair Jenkins said the campaign was well-positioned for success and would focus on winning over undecided voters in the final 100 days with the message that Scotland is wealthy enough to go it alone and could provide more jobs, better pensions and improved childcare in the event of independence.

In a speech in Edinburgh today, Mr Jenkins will argue: "In only 100 days we will be presented with the greatest opportunity our nation will ever have to create the kind of country we all know Scotland can and should be. It is a prize we must do ­everything in our power to secure for the people of Scotland."

He will add: "I know there are many people who remain to be convinced, and it is by talking with them in millions of conversations that we will win them over to Yes.

"Over the next 100 days I appeal to every single person who believes in Yes to make it their business to talk with and persuade those who remain undecided to come our way."

In other developments yesterday, Lord West, former First Sea Lord and Labour security minister said independence posed "the greatest grand strategic threat to the security and defence of our islands".

He added: "There is no doubt whatsoever that if Scotland separated it would diminish our ability to defend these islands."

Meanwhile Sir Nicholas Macpherson, Permanent Secretary to the Treasury, conceded claims about the cost of setting up the institutions required to run an independent Scotland had been "mis-briefed".

He was referring to claims in a recent Treasury report, widely leaked in advance, which suggested the cost could be as high as £2.7 billion.

The figure was later rejected by the academic on whose work it was said to be based.

Sir Nicholas repeated previous warnings about the SNP's plan for a currency union between an independent Scotland and the rest of the UK, claiming: "A small country in a currency zone when things go wrong isn't a happy place to be"