The British government set out conditions on Tuesday under which Scotland would be allowed to hold a referendum on breaking away from the rest of Britain, a move which it strongly opposes.
Any referendum must be limited to a straightforward yes-or-no question on independence, excluding the option of asking whether Scotland should be given greater devolved powers, and should be held as soon as possible, the British government said.
The Scottish National Party, which heads a devolved government in Edinburgh, has said it plans to hold a referendum by 2016 on ending the 300-year union with England.
But the British minister responsible for Scotland, Michael Moore, said London believed that the Scottish parliament did not have the legal authority to hold an independence referendum.
It is essential that the referendum is legal, fair and decisive. As a government, we have been clear ... that we will not stand in the way of a referendum on independence. But neither will we stand on the sidelines and let uncertainty continue, Moore told parliament.
The British government said it would allow the Scottish parliament to hold a referendum but there must be a single straightforward question and that question must be asked as soon as possible.
Scotland's First Minister Alex Salmond, has suggested a referendum could be held between 2014 and 2016, wanting time to rally support for Scottish independence.
But Prime Minister David Cameron, who wants to keep the United Kingdom intact, says that delay creates uncertainty that is damaging investment in the Scottish economy and wants a referendum sooner rather than later. British officials suggest a referendum could be held within 18 months.
Cameron also wants to avoid muddying the issue by asking voters any question about giving Scotland more devolved power.
Salmond, speaking before the government's announcement, said on Tuesday he was sceptical about Cameron's intervention.
The only concern of that Conservative prime minister is he wants to hold Scotland in an iron grip and the message from Scotland is 'I'm sorry, these days are over', he told BBC radio.
Scotland, which kept its own legal system after the 1707 union, has had a devolved government since 1999, with control over health, education and prisons in the nation of five million.
But London says only the British parliament can decide on Scotland's relationship with the rest of Britain.
The SNP won an overall majority in Scottish elections last May and has promised to hold a referendum in the second half of a parliamentary term lasting until 2016.
Some reports say the party hopes to exploit two events in 2014, the Commonwealth Games in Glasgow and the 700th anniversary of the Battle of Bannockburn when the Scots vanquished an English army, to create momentum for a breakaway.
Scotland has many of the trappings of an independent nation - its own flag, sports teams and a history of achievements in science and literature.
The SNP argues that Scotland would prosper as a small country in its own right. Salmond told Reuters last year that Scotland would be entitled to the lion's share of North Sea oil revenues if it went its own way.
He also wants Scotland to have its own forces and foreign policy, rejecting nuclear submarines based close to Glasgow.
An Ipsos MORI poll last month found that, among Scottish voters certain to vote in a referendum, 38 percent would vote for full independence, up three points from August, while 58 percent were opposed.
The centre-right Conservatives are weak in Scotland, having only 15 seats in the 129-member Scottish parliament.
However, the Conservatives, their Liberal Democrat allies in the coalition and Labour all oppose Scottish independence which they say would weaken both Scotland and Britain.
(Additional reporting by Matt Falloon; editing by Keith Weir)