A deal to head off a constitutional crisis over plans for a referendum on Scottish independence was emerging as the Conservatives, Labour and the Liberal Democrats joined forces to oppose the break-up of the United Kingdom.

The British Government may allow Alex Salmond, the First Minister and leader of the Scottish National Party, to hold a referendum on independence in 2014 in exchange for a straight "yes" or "no" question in the ballot.

Salmond would be forced to drop his planned third option which would allow for the devolution of all powers, except foreign and defence policy, from London to Edinburgh. Known as "devo max", it is seen as an insurance policy if full independence is rejected by the Scottish people.

In London, ministers said the devo max question was a "red line" they would not cross. They will insist on a ballot in which Scots would choose between independence and remaining in the Union, even if this means a protracted legal wrangle which may have to be resolved by the Supreme Court.

Although pro-Union politicians would prefer to see the referendum held next year to end the uncertainty about Scotland's future, ministers are ready to show flexibility over the date if Salmond abandons the devo max option. Senior Labour and Liberal Democrat figures believe Prime Minister David Cameron was "inept" at the weekend in the way he demanded a vote "sooner or later", allowing Salmond to accuse London of trying to hijack the process.

The Scottish First Minister says: "Whatever people's views on independence, there is nothing more guaranteed to antagonise the average person in Scotland than a Tory Prime Minister lecturing the democratically elected Government of Scotland on how a fair referendum should be run."

However, Labour and Liberal Democrat sources say they are pleased Cameron's move has finally sparked a debate about the merits of independence - and could force Salmond to answer detailed questions about how Scotland would look if it stood alone.

The Conservatives are deeply unpopular in Scotland and the Liberal Democrats lost support north of the border after entering the Coalition in 2010.

The SNP is due to publish proposals for its 2014 poll by the end of the month. A further flashpoint with London is the suggestion that Scots aged 16 or 17 should be able to vote.