Two referendums on independence?

Mr Salmond said the referendum is likely to be held "towards the end of the four-year term" at a cost of around 7 million. The ballot would ask voters if they agree or disagree with negotiating a new settlement with the British government based on a white paper so that Scotland becomes an independent state.

But because of the limited powers of the Scottish Parliament, the 2010 referendum will be able to ask the Scottish people only if they are willing to begin negotiations on breaking away from Britain - rather than a straight question on whether they are in favour of independence per se.

Academics predicted this will lead to Westminster, which is ultimately in charge of the constitution, calling a further referendum - at additional cost - on whether Britain wants to go ahead with any independence settlement.

They said it was certainly "extremely unlikely" Scotland will achieve independence within the first four years of an SNP-led government. However, the SNP yesterday insisted the single referendum, which would be based on a white paper setting out the terms of a settlement, will determine the future of Scotland "smoothly and swiftly" in the same way as the devolution settlement.

The Labour Party pounced on the possible delays in the process of independence as proof of the uncertainty an SNP-led government could bring. The issue of independence remained the main weapon of the ruling party with Gordon Brown, the Chancellor, and Lord Robertson, the former head of NATO, both attacking Mr Salmond over the weekend.

However, former first minister Henry McLeish said Scottish Labour should be willing to consider more powers for the Scottish Parliament in order to distance themselves from Westminster and prove they can stand up for Scotland.

With the SNP leading in the polls, the timing and cost of an independence referendum has become a central issue of the Scottish elections in May. Mr Salmond said he wanted a referendum on Scottish independence by 2010 because the party wanted to "build up credibility in government".

However, John Curtice, professor of politics at Strathclyde University, said a referendum at that late stage would mean dragging the issue into the next Scottish election because even if the answer is 'yes', negotiations on independence are unlikely to be finalised within the year. Furthermore, he said it was likely the Westminster government will call a second referendum on the agreed settlement.

He added: "It seems extremely unlikely if the SNP were to win the referendum and the UK government were willing to negotiate, that Scotland would secure independence in the next four years."

And he warned: "The SNP may have to win a second election."

Allan Trench, of the constitution unit at University College London, also said a single referendum was unlikely to resolve the issue of Scotland's future. He said a second referendum could be called on whether an agreed independence settlement should go forward.

"I think a single referendum would be bad practice and undesirable from the point of view of the Scottish people because it means they do not actually know whether independence will come or not, and under what terms. It gives too much power to the government to decide these questions and not enough to the people. A second referendum would be a better way forward for the UK and for Scotland."

However, the SNP insisted one referendum, that sets out the detailed terms of an independence settlement, would suffice, as, if it was successful, the people would have decided the way forward for Scotland. A spokesman said: "We take the view that one referendum would settle the issue as was the case for devolution."

Any doubt hanging over a referendum would open up the possibility of a coalition with the Liberal Democrats, who have refused to work with a party committed to independence.

However, Nicol Stephen, the leader of the Scottish Liberal Democrats, said his party would remain out of the "constitutional treacle" in favour of policy issues ands added:

"The SNP won't get anything done until they have had a fight with London."

Andy Kerr, the Health Minister, said the uncertainty of impending independence would irrevocably damage Scotland.

"Why would any business locate in Scotland over the next five years with the constant threat, uncertainty and risk of independence hanging around?" he asked. "With the SNP, the cost and uncertainty begins on May."

Colin Fox, the leader of the SSP, warned that leaving a referendum too late would give Westminster time to prepare opposition.