THE Scottish Government will not make a cast-iron pledge to keep the pound as part of a currency union with the UK when it publishes its detailed plan for independence later this month.

A senior official has revealed that the White Paper, the Scottish Government's blueprint for the new state, will acknowledge that Alex Salmond's key economic policy would depend on negotiations with the UK Government.

Colin McKay, the head of the Scottish Government's strategy unit, also warned the SNP's proposed 18-month transition timetable for independence would be impossible unless the UK Government wanted to negotiate a smooth transfer of powers.

Details emerged as MSPs prepared to approve legislation today paving the way for the referendum.

Mr McKay, one of the most senior civil servants involved in preparing the eagerly awaited White Paper, was speaking at a seminar organised by the Scottish Constitutional Futures Forum of constitutional lawyers.

Throughout the referendum debate Mr Salmond has stated an independent Scotland will keep sterling in a currency union with the UK, arguing the arrangement would be in the UK's best interests.

But Mr McKay said different levels of conditionality would apply to the various plans and aspirations for an independent Scotland set out in the document. It would include some cast-iron plans, he said, and other proposals which the Scottish Government was committed to achieving in negotiations.

Giving the example of the proposed currency union - a plan to share the pound and the Bank of England - he said: "We cannot assert as an a priori fact we can achieve a currency union with the UK, but we can set out why we think it is the best option." He said other White Paper proposals would take the form of political promises.

He gave the example of abolishing the bedroom tax which, he added, was "not an inevitable consequence of ­independence". Mr McKay also said the Scottish Government would use the White Paper to "indicate support" for measures it would like to see agreed by others, particularly in the proposed written constitution of an independent Scotland.

Alex Salmond has already suggested enshrining free education for all - a key SNP policy - in a written constitution but has accepted the bill of rights should be drawn up by a commission independent of government or parliament.

On the SNP's plan for Scotland to formally leave the UK in March 2016, after 18 months of talks, he warned it was "an impossible timetable" if Westminster refused to embrace a speedy transition of powers. But he added: "The assumption of the Scottish Government is that given the Edinburgh Agreement says both sides will respect the outcome of the referendum, it will be in the UK's best interests to negotiate a smooth transition and ensure we remain in Europe."

The document, running into hundreds of pages, will be published on November 26.

Scottish Secretary Alistair Carmichael has said the SNP Government must answer key questions on currency, pensions and cost in the paper. He also echoed UK Chancellor George Osborne by casting doubt on whether the UK would agree to share its currency with an independent Scotland.