Scottish independence referendum: Labour, Tories and Lib Dems unite to promise more power to Holyrood after 'no' vote

The leaders of the main opposition parties in Scotland also said they would work to boost Holyrood's responsibility for social security after a No vote in September’s referendum (Getty)

Labour, the Conservatives, and the Liberal Democrats have issued an unprecedented joint pledge to increase the tax and spending powers of the Scottish Parliament if Scots vote against independence.

Johann Lamont, Ruth Davidson and Willie Rennie, the leaders of the main opposition parties in Scotland, also said they would work to boost Holyrood's responsibility for social security after a No vote in September’s referendum.

Their declaration came after criticism from Nationalists that the pro-Union parties could not agree on their vague pledges of further devolution. The three leaders said: "We now pledge to further strengthen 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."

They added: “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."

Welcoming the statement, David Cameron said: "This demonstrates beyond any doubt that the people of Scotland can be assured that the powers of the Scottish Parliament will be enhanced whilst retaining the strength, stability and security of the United Kingdom."

Ed Miliband said a Scotland Bill in the first Queen’s Speech of an incoming Labour government would guarantee more powers for Holyrood. “That is a firm promise to the people of Scotland,” he said.

But Blair Jenkins, chief executive of the Yes Scotland campaign, claimed the pro-Union parties "know they are losing the argument and that is the only reason they are now talking about more powers for Holyrood". He said Westminster would make any decision on enhanced devolution.

"The lessons of the past demonstrate quite clearly that Westminster cannot be trusted to deliver on the promises they have made on extending Scotland's democracy. All three main Westminster parties have form on this,” he said.

The first draft constitution of an independent Scotland was published yesterday by the Scottish Government. It includes plans to remove Trident nuclear weapons from the country and keep the Queen as head of state, and says that the Scottish people are “sovereign”, rather than Parliament or Government.

In the event of a Yes vote, there would be a "temporary" constitution, to be replaced later by a permanent one. Nicola Sturgeon, Scotland's Deputy First Minister, unveiled draft legislation which, if enacted, would take effect March 24, 2016, to be known as “independence day”.