Media playback is unsupported on your device Media caption The future of Scotland should be decided by the Scottish people, says Nicola Sturgeon

David Cameron has said he is not trying to "dictate" the terms of a Scottish independence referendum.

Government sources have denied reports Westminster would set an 18-month limit on holding the poll, if it transfers powers for a binding vote to Holyrood.

But ministers may insist on a "yes or no" vote on independence - without the option of more powers for Scotland.

Scotland's Deputy First Minister Nicola Sturgeon accused Westminster of "trying to interfere in Scottish democracy".

But Mr Cameron said he wanted the vote to be "legal, fair and decisive".

Advisory only

The Scottish National Party has pledged to hold a referendum in the latter half of its term, with 2014 thought to be the preferred date.

The UK government is expected to say that Scotland can hold a referendum on any subject it chooses but the result would only have advisory status and could be open to legal challenge.

The scottish secretary will say a vote organised by the Scottish government would be purely advisory and open to legal challenge since the power to change Scotland's constitutional status rests with Westminster Scottish secretary to brief MPs

It is expected to confirm that Westminster could formally transfer additional powers to Holyrood to allow it to hold a binding referendum.

But BBC political editor Nick Robinson said conditions could be attached to that transfer, including ruling out a third choice in any vote - for greater powers to be devolved to Scotland without full independence - which Mr Cameron fears could split the unionist vote.

Scottish Secretary Michael Moore will make a statement to the Commons on Tuesday, he added, outlining the government's opinion on the legal status of an independence referendum.

It had also been reported that Downing Street could set a time limit, possibly 18 months, on any binding vote, but government sources have denied this.

'Decisive question'

Mr Cameron told Sky News he wanted to resolve that "legal uncertainty" and wanted to work with the Scottish government to give the people of Scotland the option of "a fair and more decisive question", put earlier rather than later.

"But we're not going to dictate this, this is something we want to resolve, the legal position," he added.

Media playback is unsupported on your device Media caption David Cameron: "I strongly support the United Kingdom"

But amid reports of a possible time limit, Scotland's Deputy First Minister Nicola Sturgeon accused Mr Cameron of "a blatant attempt to interfere" in a decision that should be for the Scottish government and Scottish people.

"It's the attachment of conditions that gives the game away - this is Westminster trying to interfere," she told BBC Radio 4's Today programme.

"Perhaps I should be relaxed about that because the more a Tory government tries to interfere in Scottish democracy then I suspect the greater the support for independence will be, but there is a key issue of democratic principle here.

"The SNP was elected on a clear prospectus and it's right that now that we have the mandate we can proceed on that basis."

Legal challenge

The prime minister's spokesman said the 1998 Scotland Act, which brought about devolution, made clear that constitutional issues were reserved for Westminster.

"Clearly, a number of independent commentators and legal experts have highlighted the fact that a referendum Bill passed by the Scottish Parliament could be open to legal challenge," he added.

Media playback is unsupported on your device Media caption The BBC's Lorna Gordon has been gauging reaction in Scotland to the referendum

Ms Sturgeon said referendums in the UK were always "consultative and advisory" so talk of a binding vote was unnecessary.

She said the SNP would prefer a straight yes/no referendum, but there was "a significant body of opinion" in Scotland which was in favour of financial independence, but not full political independence.

The Scottish government's website states that, even if a non-binding referendum was held: "The moral and political force of a vote for independence would be enormous, and impossible for a future government to ignore. A negative vote would similarly have a political consequence."

'Resentment'

But Conservative peer Lord Forsyth, a former Scottish secretary, said the SNP had not set a firm date for a referendum because they knew the majority of people were currently opposed to full independence and "they are afraid they will lose it".

"They want to spend the next two or three years creating resentment on both sides of the border," he told BBC Radio 4's World At One.

"It is quite extraordinary that the party which stands for independence for Scotland won't take yes for an answer from David Cameron when he says the Scottish people can decide".

Chancellor George Osborne led discussions on Scotland at Monday's cabinet meeting and set out his concerns about the impact of uncertainty over a referendum on Scotland's economy. The PM's spokesman said both Mr Cameron and Mr Osborne had been told in private by business leaders that uncertainty was deterring inward investment.

Scottish Labour leader Johann Lamont told the BBC a vote should take place "as soon as possible" on "a clear question".

"It's necessary for the people of Scotland to be given the opportunity to decide their constitutional future sooner rather than later in order that the uncertainty around the economy, around business and all the rest of it is addressed," she said.