By Alex Stevenson

A Scottish independence referendum can only take place under terms approved by London, the coalition government has said.

Scottish secretary Michael Moore issued a consultation paper making clear that the UK government was keen to establish a referendum which was legal, fair and decisive – but attaching its own conditions to the vote by doing so.

A referendum would only be legitimate if it took place within a specific timeframe, thought to be within 18 months, and only offered two choices to the voter.

The latter would block a third 'economic devolution' option thought to be favoured by the SNP.

If the nationalists in power in Holyrood refused to accept the proposals the UK government would force it upon them by amending the Scotland bill currently working its way through parliament, today's consultation revealed.

Mr Moore wrote in the foreword to the consultation paper: "We think it is in nobody's interests to have a referendum process subject to a legal challenge with all of the delay and uncertainty that would entail.

"We are optimistic that we can reach a conclusion that will be fair and legitimate to both sides of the debate and people in Scotland as a whole."

The Electoral Reform Society Scotland spoke out against the UK government's position, arguing that the Scottish government had a "stronger mandate" to set the terms of an independence referendum.

"Both sides clearly want to fight this referendum on terrain most favourable to them. The key test here can't be about which side benefits most from setting the question or date – but on which body has the greatest democratic legitimacy to do so," ERS Scotland director Willie Sullivan said.

"It would seem the Scottish parliament has the stronger mandate to debate and decide the referendum proposition and timing, with the Electoral Commission ruling on the final question and the operation of the referendum.

"We acknowledge there are legal issues to be addressed so it's essential both governments ensure any vote is not open to obvious legal challenge."

Earlier Labour leader Ed Miliband said he backed the "notion of greater clarity", in a tacit acceptance of the coalition government's approach to the issue.

"Labour's been saying for months, name the date and name the single question and get on with it," the leader of the opposition said.

"In the end it's for the people of Scotland to express their will about the kind of clarity required.

"I think a cross-party campaign will be led by people of grit and steel right across the political spectrum. There will be lots of people involved in the campaign, including me."