Gordon Brown’s closest political ally has urged Labour MPs to show “some backbone”, amid speculation that the PM could face a leadership challenge. The knives have been out for the prime minister in the wake of the disastrous by-election loss of Glasgow East to the SNP.

Ed Balls, schools secretary, said: “What we have got to show is the strength, the discipline, the backbone as a political party to do what the public wants.”

The call was backed by justice secretary Jack Straw, who MPs see as the future leader of a delegation of cabinet ministers to order Mr Brown to leave his position. “It would be a big mistake for the Labour party to now turn in on itself and indulge in a summer of introspection,” Mr Straw said.

It would appear that senior Labour figures both inside and outside the cabinet are co-ordinating their efforts to remove Mr Brown from power when the political season starts again in September.

One senior MP said that there was a “plan” under which the PM’s critics would hold their fire until September, at which point they hoped a large number of Labour MPs would ask that the prime minister had to go.

In front of senior officials at Labours National Policy Forum in Warwick on Friday, Mr Brown was accused of a “lacklustre” performance that did little to raise morale. Mr Brown blamed his party’s problems on the global economic downturn – a tune he has sung many times before.

It was bloody awful, he can’t change the script,” said one of those at the gathering of ministers, union leaders and constituency activists. “If that’s the best he can do, then God help us,” said another.

The three-day meeting ended with a policy agenda for the next election that includes lowering the voting age to 16 and a fully-elected House of Lords.