Cameron Miliband Clegg
David Cameron (left), Nick Clegg (centre) and Ed Miliband (right) have all promised Scotland more devolution if it rejects independence.Getty

The leaders of the Conservatives, Labour and the Liberal Democrats have all signed a pledge to devolve more powers to Scotland if it votes 'No' in the independence referendum.

David Cameron, Ed Miliband and Nick Clegg all put their names to the "The Vow, which has been published on the front page of Scotland's Daily Record newspaper.

It is a last ditch attempt to convince Scottish voters in the knife-edge referendum that their country would be better off staying in the UK, with polls suggesting there is little to split the 'Yes' and 'No' campaigns.

Among the promises are "extensive new powers" for the Scottish parliament. And that the final say on how much is spent on the NHS will be a matter for the Scottish government, which is allocated resources from central UK government under the Barnett formula and can also raise money itself.

"We agree that the UK exists to ensure opportunity and security for all by sharing our resources equitably across all four nations to secure the defence, prosperity and welfare of every citizen," says the pledge.

"We believe that the arguments that so powerfully make the case for staying together in the UK should underpin our future as a country.

"We will honour those principles and values not only before the referendum but after. People want to see change. A 'No' vote will deliver faster, safer, and better change than separation."

But Alex Salmond, leader of the Scottish National Party (SNP) and campaigner for a 'Yes' vote, has said the best way for Scotland to ensure it has more powers is to back independence.

Scotland will vote on 18 September.