Graham Brady, a senior Conservative lawmaker and chairman of the party’s powerful “1922 committee”, urged Cameron, who has pledged to renegotiate Britain’s EU ties before holding a referendum by end-2017, to respect the “deeply held views of others” in order to keep his fractious party united.

“I would urge the Government to treat EU membership as a matter of conscience for front and backbenchers alike,” Brady wrote in The Daily Telegraph newspaper.

“It must be in the interests of the Conservative Party to treat the referendum campaign as a discrete issue, one that needn’t divide us on the vast majority of the Government’s program for government on which we are entirely united.”

Brady’s intervention comes after Cameron tripped up over Europe at a summit of the Group of Seven Industrial nations (G7) on Monday, appearing to issue an ultimatum to his own ministers over the EU only to swiftly withdraw it, saying he had been misunderstood.