Westminster paves way for 'legal and decisive' Scottish independence referendum. Press Association

The government has published proposals to pave the way for a referendum on independence for Scotland.

A draft order drawn up in Westminster would temporarily extend the powers of the Scottish parliament to allow it to call a referendum on the single question of whether or not Scotland should remain part of the United Kingdom.

The Scottish deputy first minister, Nicola Sturgeon, said the Scottish government wanted to hold the referendum in autumn 2014.

Legal advice received by the government suggests any attempt by the Scottish government to mount a referendum on the basis of an act passed at Holyrood would be unlawful and would be struck down by the courts.

A consultation paper published by the Scotland secretary, Michael Moore, proposes a Section 30 order to give Holyrood the power to deliver a referendum. The order would have to be approved by both houses at Westminster and by the Scottish parliament.

Officials said this method would allow a referendum to be staged within 12 to 18 months if all parties co-operated, but the question of whether a legal deadline should be included in the order will not be decided until after the consultation concludes, on March 9.

Ministers said they wanted any poll to be held "sooner rather than later", to reduce uncertainty, which they believe is damaging the Scottish economy.

In a foreword to the consultation document, the prime minister, David Cameron, and his deputy, Nick Clegg, made clear that they did not want the break-up of the union, but said it was right that the people of Scotland should be allowed to vote on whether they wanted to remain part of the United Kingdom.

Cameron and Clegg acknowledged that the Scottish National Party fought and won last year's Scottish parliament election on a manifesto promising an independence referendum by the end of the current term, in 2016.

Cameron and Clegg wrote: "They [the SNP] have campaigned consistently for independence, and while the UK government does not believe that this is in the interests of Scotland or of the rest of the United Kingdom, we will not stand in the way of a referendum on independence.

"The future of Scotland's place within the United Kingdom is for people in Scotland to vote on."

They added: "Ending Scotland's place within the United Kingdom is not the policy of the UK government but we owe it to everyone in Scotland to ensure that the referendum is delivered in a legal, fair and decisive way."

The order proposed would require any referendum to be held under the terms of electoral laws passed at Westminster in 2000, under the oversight of the Electoral Commission.

There would be only one ballot paper, offering voters a single choice between the two options of independence or remaining part of the UK. All those registered to vote in Scottish parliamentary elections would be entitled to vote, and the poll could not be held on the date of any other referendum.

Today's paper also set out the alternative of using legislation at Westminster to authorise the referendum, possibly by an amendment to the Scotland bill, which is currently going through parliament.

But it made clear that the legal opinion of the UK government is that, without either of these measures, the Scottish parliament does not itself have the power to pass legislation to trigger a referendum.

The Scotland Act of 1998, which established devolution, states explicitly that the question of "the union of the kingdoms of Scotland and England" will remain a matter reserved to Westminster.

And it says that any act passed in Holyrood on reserved matters is "not law" because it is outside the legislative competence of that parliament.

The attorney general and the advocate general for Scotland are required by law to consider all legislation passed at Holyrood within four weeks and decide whether to refer it to the supreme court in London for a ruling on whether it falls outside the Scottish parliament's powers.

But in practice, ministers believe individuals in Scotland would be "very likely" to refer a referendum act to the supreme court themselves before this process is complete.

"It is the UK government's view that any bill introduced in the Scottish parliament providing for a referendum on independence would be outside the powers of the Scottish parliament and, if challenged, would be struck down by the courts," said the consultation paper.

And it added: "The UK government does not believe that it is in Scotland's interests to have Scotland's constitutional future decided in court. The referendum must be legally watertight. There must be no doubt that it is lawful.

"That certainty can only be provided by legislation involving the UK parliament."

For the referendum to be considered fair, its rules and oversight must be "manifestly and overtly above board", said the consultation paper.

"We cannot allow the possibility that the result of the referendum is questioned because the rules were considered unfair."

The paper also warned of the danger that a referendum with more than two options could fail to provide a decisive result.

"The referendum must be decisive," it said. "It is the UK government's view that for this to happen, there must be a single, straightforward question and that question must be asked as soon as possible.

"We live in uncertain times, with the global economic situation creating challenges for increasing investment and jobs in Scotland. The question of Scotland's constitutional future is increasing that uncertainty. It is irresponsible to allow this question to hang over Scotland, when it is in our power to end the doubts and allow Scotland to move forward with a clear constitutional future."