William Hague has said Douglas Carswell's decision to defect to Ukip is "totally counter-productive" because only the Conservatives can offer the public a choice on EU membership.
The Leader of the Commons said: "It's a totally counterproductive thing to do, of course, because the only chance of real change in Europe, the only chance of having a referendum where people can decide to say in the EU or leave the EU is having a Conservative government."
Backbench Tory MP Mark Pritchard agreed with Mr Hague, saying Mr Carswell had made an "error of judgment" that would end up helping Labour in the 2015 general election.
More top news
A Branislav Ivanovic header in extra-time ensured that Chelsea are through to the Capital One Cup Final after beating Liverpool
Scientists have discovered the oldest known earth-size planets discovered which they say raises the prospect of 'ancient life'.
Auschwitz survivor Alina Dabrowska was a victim of the cruel experiments of Nazi doctor Josef Mengele, known as "the Angel of Death."