The leaders of the three main pro-Union parties in Scotland have come together to pledge Holyrood will get more powers in the event of a No vote in the independence referendum.

As revealed in The Herald last week the public commitment made heavy use of the word "guarantee" but there was little fresh detail on show as activists on the steps of the National Monument in Edinburgh held up letters proclaiming "More Powers for Scotland Guaranteed."

Ben Thomson, chairman of think tank Reform Scotland, advocates of a more radical Devo Plus approach, had called for the three parties to pool the best parts of their proposals. "While we would wish them to go further, the best bits of all three would be a radical step towards the real accountability proposed by Devo Plus," he said.

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Instead each of the parties promises only to include its proposals in its own manifesto for the next General Election.

The statement signed up to by Johann Lamont, Ruth Davidson and Willie Rennie yesterday said: "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.

"Our common endeavour will deliver a stronger Scottish Parliament in a stronger United Kingdom."

The pledge also set out a vow to strengthen further the powers of the Scottish Parliament, in particular in the areas of fiscal responsibility and social security.

"We believe that Scotland should have a stronger Scottish Parliament while retaining full representation for Scotland at Westminster. Our common agenda can bring people together from all of Scotland, from civic society and every community," it added.

Prime Minister David Cameron claimed the statement demonstrated "beyond any doubt" that the people of Scotland can be assured that the powers of the Scottish Parliament will be enhanced while retaining the strength, stability and security of the United Kingdom.

UK Labour leader Ed Miliband said: "If elected prime minister, in less than a year's time my government will include a Scotland Bill in its first Queen's speech, guaranteeing more powers for the Scottish Parliament. That is a firm promise to the people of Scotland."

Blair Jenkins of Yes Scotland claimed the pro-UK parties "know they are losing the argument and that is the only reason they are now talking about more powers for Holyrood".

This was rejected by Ms Lamont who said: "We want to come together, build a consensus, to be clear that the choice on September 18 is not between change and no change. We are guaranteeing the Scottish Parliament will have more powers."

Ms Davidson claimed: "The Nationalists are probably genuinely apprehensive about the fact that we've been able to come together in this way, because they never expected we would or we would be so enthusiastic about it."

Mr Rennie insisted: "Our proposals are the most popular constitutional change on the agenda, more popular than independence.

"There are many common strands between all the proposals, so people know that there will be more financial responsibility, there will be more social security responsibility. "