Meanwhile International Development Secretary Hilary Benn appeared to be establishing an advantage over his rivals in the race to replace John Prescott.

Mr Benn was confirmed as bookies' favourite after Labour revealed that he had secured the nominations of 77 constituency parties - more than any of his rivals.

But Labour's complicated electoral college system makes it virtually impossible to judge whether this lead will translate into victory when votes are counted at a special conference in Manchester on June 24.

Backbencher Jon Cruddas was endorsed by 68 constituency Labour parties, with justice minister Harriet Harman on 60 and Education Secretary Alan Johnson 45.

Labour chairwoman Hazel Blears and Northern Ireland Secretary Peter Hain lagged behind on 36 and 23 respectively.

But activists are not obliged to vote in accordance with their constituency party's recommendation, and anyway, the rank-and-file members have only a third of the vote in the election, with unions and affiliated organisations taking another third and MPs and MEPs the remainder.

Mr Benn struggled more than any of the other candidates to secure the nominations of the necessary 45 MPs and he has also failed to net the same degree of union backing as rivals like Mr Cruddas and former union boss Mr Johnson.

Mr Cruddas has received nominations from the giant Amicus and TGWU unions as well as the Socialist Health Association, while Mr Johnson enjoys the backing of the CWU - which he once led - and public sector giant Unison, while Mr Benn's sole union endorsement came from Unity, which has just a few thousand eligible voters.

Confusing the picture further, union members will vote as individuals and are not obliged to follow the recommendation of their executives.