Gordon Brown on Monday night came under intense pressure from his own MPs to change the law so that the Conservatives could not benefit from the millions of pounds expected to be poured into target constituencies by Lord Ashcroft, one of the party’s biggest backers.

At a meeting of the parliamentary Labour party, several backbenchers lobbied the prime minister openly to tackle what they called a “loophole” that enabled Lord Ashcroft to bankroll spending outside of election campaigns – one of the factors that had been cited by those Labour figures in favour of an early election.