David Cameron is to take the high-risk gamble of offering to hold a referendum on Scottish independence next year. He aims to call Alex Salmond's bluff by challenging him to stage a vote on Scotland's place in the UK within 18 months.

Mr Cameron will this week signal the start of a bruising battle with the Scottish Nationalist First Minister over the future shape of the UK.

The Prime Minister will publish legal advice that concludes the SNP administration in Edinburgh can hold a binding referendum only with the British Government's permission. He will say he is prepared to give his backing to a vote only as long as it is held by the summer of 2013.

Mr Salmond wants to hold the referendum in the autumn of 2014 to capitalise on the patriotic buzz caused by the Commonwealth Games and the Ryder Cup, both of which will be held in Scotland that summer.

A spokesman for Mr Salmond insisted he was not prepared to enter into deals. Mr Cameron says he will back a vote only if it is held next year. He is set to publish legal advice stating that Scotland can carry out a binding referendum only with the backing of the UK Government.

Mr Salmond's Scottish National Party (SNP) said it had never argued that the referendum was going to be "legally binding". It would, however, be "politically binding" because it would represent the views of the Scottish people and be impossible for any government to resist. Ministers in Mr Cameron's Coalition hope that an earlier vote would enable campaigning for a referendum to take place in the afterglow of the Queen's diamond jubilee celebrations and the London 2012 Olympic Games.

Under Mr Cameron's plan, Scottish voters would only be offered a simple "yes/no" question on independence, whereas Mr Salmond wants to include another "independence-lite" option of greater financial autonomy short of complete separation.

UK ministers sought yesterday to wrest the political initiative from Mr Salmond by arguing that the uncertainty about the timing of the referendum, and its exact form, was damaging the Scottish economy and unfair to the Scottish public. Mr Cameron told BBC1's The Andrew Marr Show: "I don't think we should just let this go on year after year. I think that's damaging for everyone concerned, so let's clear up the legal situation and then have a debate about how we bring this to a conclusion. My view is that, sooner rather than later, would be better." Danny Alexander, the Chief Treasury Secretary and Liberal Democrat MP for Inverness, Nairn, Badenoch and Strathspey, said: "Scotland cannot afford a long period of dithering and delay from Alex Salmond."

Ministers in London privately admit that they have failed to take the fight about the UK's future to Mr Salmond since the SNP won a dramatic election victory last May. They have not be helped by the turmoil of the three main pro-Union parties, with the Tories, Liberal Democrats and Labour all electing new leaders in recent months.

Under Mr Cameron's move, the Government atWestminster would insert a "sunset clause" into the Scotland Bill, which is before the House of Lords, allowing the Scottish Government to hold the referendum in the next 18 months. But SNP sources said Mr Salmond was not prepared to enter in deals on a referendum and would veto Mr Cameron's "sunset clause" if Westminster tried to impose it on Scotland.