Senior Conservative backbenchers have called for the early break-up of the coalition after research suggested Tories could lose as many as 12 priority seats in next year's general election to Ukip, leaving David Cameron short of an overall majority.
If performances at this month's Newark by-election are repeated at the poll next May, the Bow Group think-tank calculated that Nigel Farage's party could snatch a dozen constituencies from the Tories "40/40" list of the 40 most vulnerable seats they are defending and the 40 they hope to gain from other parties.
Under this analysis, Conservaties would win just 47 of the seats and Labour 21.
The Bow Group warned that it would be "very difficult" for Mr Cameron to win next year without a "major shift" on issues like Europe and immigration, on which tensions with Liberal Democrats in the coalition are strongest.
Without a change in direction from the Conservatives, Ukip is on track to win seats - most of which will come from the Tories - while Labour would be "significantly more supported than constrained" by the gains made by Mr Farage's party, the group said.
The chairman of the influential Conservative backbench 1922 Committee, Graham Brady, said: " I don't think the coalition should last right up to 2015. I've made the point that I think both parties will need some space, some independence, so that they can present their separate visions to people before the General Election."
Former cabinet minister John Redwood said: "It is vital to the interest of both Liberal Democrats and Conservatives that the Conservatives should be able to articulate their conservative message and the Liberal Democrats theirs, not being governed by the normal niceties of collective responsibility.
"There are many backbenchers who think what should happen now is that the Conservative majority within the Government should start to press very strongly for two or three conservative policies and then when the Liberals really don't like it, maybe the Liberals will wish to leave."
Bow Group chairman Ben Harris-Quinney said: "There has been much talk of listening and hearing from the Conservative Party leadership following Ukip's success in the recent European and Newark elections, and much resulting criticism of continued federalism in Europe following the candidacy of Jean-Claude Juncker for EU president, yet at home we remain in coalition with a committed euro-federalist party in the Liberal Democrats.
"Whilst this continues to be the case, it will be impossible to convince voters that the Conservative Party remains genuinely committed to conservative policies on Europe and immigration. Breaking up the coalition Government as soon as possible would send that signal, and it might be a bold enough move to give the Conservative Party a chance of a majority in 2015."
While Tories won the Newark by-election by a comfortable majority of more than 7,000, the party's share of the vote slumped by almost nine percentage points, while Ukip gained 22 points.
On the basis of this swing, the Bow Group calculates that Ukip could take control of Conservative seats Camborne & Redruth, Thurrock, Newton Abbot, Waveney, Plymouth Sutton & Devonport, and Halesown & Rowley Regis in the 2015 election, and win Tory target seats from Labour in Great Grimsby, Telford, Walsall North, Newcastle-under-Lyme and Plymouth Moorview and from Lib Dems in St Ives.