Britain's three main parties will jointly release a pledge to Scottish people that will promise their devolved parliament more power to control tax and social security, if voters reject independence.

UK Prime Minister and Conservative leader David Cameron, Liberal Democrats' Nick Clegg, and Labour's Ed Miliband will unveil a joint formal declaration that Scotland will have more "fiscal responsibility and social security" if voters decide to keep the country within the UK.

"We support a strong Scottish Parliament in a strong United Kingdom and we support the further strengthening of the parliament's powers," says excerpts of the declaration.

"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.

"This commitment will deliver a stronger Scottish Parliament in a stronger United Kingdom."

Scottish people will vote in an independence referendum on 18 September, 2014, and will be asked the straight "yes/no" question: "Should Scotland be an independent country?"

The referendum period started on 30 May. Latest polls show that the gap between a 'Yes' and a 'No' vote is rapidly closing but it is still unlikely that a union break will happen.

Meanwhile, the Scottish Nationalist Party (SNP) hit back at Whitehall's pledges.

"No one in Scotland will be fooled by this Westminster-led rehash of vague promises and unspecified more powers in the event of a 'No' vote - the Tories have tried that before," said Scotland's First Minister Alex Salmond's spokesperson.

"Cameron fought tooth and nail to keep a more powers option off the ballot paper, so how can anyone take him seriously now?"