Tory MPs are threatening to “do a Carswell” unless David Cameron toughens his stance on the European Union, as the first MP broke cover to admit that he is “tempted” to defect to Ukip.

Eurosceptics said up to three MPs are prepared to give the prime minister an ultimatum that they will jump ship unless he spells out in far more detail how he plans to renegotiate Britain’s relationship with Brussels.

Ukip sources said at least one other Tory MP has told its leader, Nigel Farage, that he is prepared to follow Douglas Carswell and defect, but only after December when it will not be necessary to hold a by-election. Another MP is said to be “seriously considering” a move to Ukip.

The warning came