Britain’s largest trade union contributed more than 40 per cent of Labour’s funding in the three months to June, prompting renewed Conservative accusations that Gordon Brown is in hock to union “paymasters”.

The Electoral Commission figures also show that David Cameron, the opposition leader, continues by far to outstrip the prime minister’s cash-raising ability. The Tories raised almost £5.6m, almost 50 per cent more than Labour’s £3.8m, and a tenth consecutive quarter that the party has attracted the most donations.