Some of the three million members of unions and other affiliated bodies who control a third of the votes will gather in Bristol to grill the six candidates.

Chancellor Gordon Brown - who will succeed Tony Blair as leader uncontested - will also appear at the event to set out his own stall.

Education Secretary Alan Johnson, justice minister Harriet Harman, Development Secretary Hilary Benn, party chairwoman Hazel Blears, Northern Ireland Secretary Peter Hain and backbencher Jon Cruddas are engaged in a closely-fought tussle to be his deputy.

On Friday night, Mr Benn became the latest to receive the backing of a union's leadership as Unity (formerly the ceramics union Catu) endorsed his campaign.

It follows his call for a new agreement between the Government and unions to cover areas such as flexible working rights and closing the pay gap.

Unity general secretary Geoff Bagnall said: "Unity is backing Hilary Benn because we want a strong voice for working people and trade unions around the Cabinet table."

Mr Cruddas has the backing of Britain's biggest trade union Unite, which is urging its 1.4 million members to support the backbench MP. Amicus and the Transport and General Workers also support him.

Shopworkers' union Usdaw backs Ms Blears, while Mr Hain boasts the support of train drivers' union Aslef, the British Food and Allied Workers Union (BFAWU) and the Union of Construction, Allied Trades and Technicians (UCATT).

The winner will be declared at a special Labour conference in Manchester on Sunday, June 24, three days before Tony Blair has said he will resign as Prime Minister.