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
Jamie Oliver turned down the offer of playing a hobbit in the movie adaptation of the book by JRR Tolkien, the TV chef has revealed.
Children are being put at risk as 11% of posts for permanent children's social workers remain unfilled, experts have warned.
A Network Rail boss who presided over the Christmas trains chaos will not receive a bonus of more than £300,000 when he leaves in February