Britain's pro-union parties were struggling on Monday to present a united front on the proposed referendum on Scottish independence amid Liberal Democrat and Labour dismay over Downing Street's handling of the announcement.

Senior Lib Dem sources criticised David Cameron for adopting a confrontational approach towards the SNP after Downing Street indicated that the Scottish parliament would be given an 18-month deadline to hold a referendum.

Michael Moore, the Lib Dem Scotland secretary, will tell MPs on Tuesday that the Westminster parliament should devolve the power to hold a legally binding referendum to Holyrood on a temporary basis. But Nick Clegg has ensured that there will be no fixed, 18-month deadline. Moore is rushing forward his statement, which he had been due to make later in the week, after a backlash against the prime minister's announcement that the coalition would call the bluff of Alex Salmond by bringing forward a referendum.

"This has not been handled in an ideal way," one Lib Dem source said. "It is right that Alex Salmond should not be given a free run. But this is a risky strategy and we have to get it right."

Labour sources expressed surprise that Downing Street had failed to brief Ed Miliband, whose party is the largest pro-union party in Scotland. The sources also expressed astonishment that George Osborne briefed the cabinet on Monday on the government's thinking at a meeting of the cabinet convened at the site of the London Olympics. "It is totally bonkers for the cabinet to have been briefed on this by George Osborne at the site of the London Olympics," one Labour source said. "It is bizarre that the Scotland secretary did not take the lead in briefing the cabinet."

Labour also expressed surprise that Lord Forsyth of Drumlean, the Tory former Scottish secretary, who campaigned vigorously against the Scottish parliament in 1997, was allowed to take to the airwaves. "Michael Forsyth should not be anywhere near this campaign," the Labour source said. "He is a hate figure in Scotland. This is absolutely crackers. The iconography is appalling."

Moore will attempt to clarify the government's thinking on Tuesday when he tells MPs that the coalition is planning to give the Scottish parliament temporary powers to hold a referendum. Under the 1998 Scotland Act, which established the parliament, the constitutional future of Scotland was designated a "reserved" power which remained at Westminster. The government is planning to invoke an order under section 30 of the act to give Holyrood the right on a temporary basis to hold a referendum on Scotland's constitutional future. The Lib Dems have fought off an attempt by the Tories to set an 18-month deadline. But ministers will make clear that they would like the referendum to be held by 2013.

Westminster will not set the question in the referendum. But the referendum would be limited to a simple yes or no on whether Scotland should remain in the UK, preventing Salmond from asking a third question known as "devo max". This would give voters the option to support a souped-up version of devolution which would just fall short of independence.

Moore decided late last year to press for an early referendum after Salmond declined, in a series of meetings since his outright victory in the Scottish elections last May, to spell out his plans for a vote on the future of the union. The SNP, which has yet to persuade a majority of Scottish voters to embrace independence, is planning to delay a referendum until 2014, possibly on the 700th anniversary of the battle of Bannockburn, the key battle in the first war of Scottish independence. Salmond also wants to offer the "devo max" question which would ensure that a major step would be taken towards independence.

Nicola Sturgeon, the SNP's deputy first minister, criticised the UK government for "a blatant attempt" to interfere in a matter that should be decided in Scotland. "We were elected on the basis of our commitment to have a referendum in the second half of this parliamentary term," Sturgeon said. "This is about Westminster seeking to interfere." But ministers in London say that the SNP manifesto for last year's Holyrood elections said nothing about a referendum in 2014. It also said nothing about a "devo max" question. The SNP manifesto said: "We think the people of Scotland should decide our nation's future in a democratic referendum and opinion polls suggest that most Scots agree. We will, therefore, bring forward our referendum bill in this next parliament."

The prime minister moderated his language on Monday when he said he was not attempting to "dictate" the terms of the referendum. "We can't stand in the way of a part of the UK if it wants to ask the question 'Are we better off outside it?' We can't stand in the way of that, but what I think the Scottish people deserve is a fair, clear and decisive question. We have to have legal clarity over who is responsible for this decision. Is it the Westminster parliament or is it the Scottish parliament? We will be setting out the legal position and trying to find a way through. We are not going to dictate on this. We have first of all got to resolve this legal uncertainty and then try to work with the Scottish government and make sure there is a fair, clear and decisive outcome."