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
The chief executive of one of the 'big six' energy companies tells ITV News a report customers are paying too much is a concern.
10 years on, George Psaradakis has given his first interview, recalling the events of July 7 2005.
A spat between the supporters of Yvette Cooper and Liz has seen a debate rage over whether a woman needs to be a mother to be a good leader.