By Steve Schifferes BBC News Online economics reporter

The battle for the leadership of the huge GMB union - the second biggest among public sector workers - has officially begun.

Over 700,000 workers will receive postal ballots in a vote which may help to determine the future of the trade union movement.

Kevin Curran, GMB Northern region leader

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The leading candidates are expected to be a former London park keeper, Paul Kenny, who now runs the union's London region, and Kevin Curran, a former boilermaker who is in charge of the Northern region.

Mr Curran is standing with Debbie Coulter, who if elected would be the first woman to become deputy general secretary.

Both say they are candidates of the "centre-left", and both have been highly critical of the Labour government's approach to the unions, and in particular the firefighters' dispute.

The ballot, the results of which will be announced on 16 April, is taking place earlier than expected due to the unexpected retirement of John Edmonds, the GMB's current general secretary.

	:	Trade unions guide
The big unions at TUC 2002

The other big general union, the Transport and General Workers Union, is also set for a bitter battle for succession next year as its leader, Bill Morris, will have to retire by October.

Labour-union relations

The election will be a crucial test for the government in its relations with the unions, after a series of battles that have left its traditionally close working relationship in tatters.

Paul Kenny, GMB London region leader

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The GMB's present boss, Mr Edmonds, led the attack on the government over its plans for private financing of public investment at the TUC and the Labour Party conference this year.

And both of the leading candidates to succeed him - London region boss Paul Kenny and Northern region boss Kevin Curran - have backed that campaign.

However, Mr Kenny has fallen out with Mr Edmonds - over a dispute involving GMB head office staff - while Mr Curran paid tribute to his role.

"John Edmonds will be a tough act to follow. His battle to keep public services out of private hands must continue," Mr Curran said.

More battles ahead

A shift to the left by the GMB would crucially affect the balance in the trade union movement.

Among the four unions that dominate the TUC, two - Unison and Amicus - already have leaders who do not share Mr Blair's reform agenda.

And new generation of leaders are not afraid of confrontation with the Labour government if they believe it is in their members' interests, and increasingly see the public sector as the main arena of conflict.

The recent fire service strike, and the militancy now on display from doctors, civil servants, local government workers and teachers, are all signs that the government may have a more difficult task in the future.

How Mr Blair handles the new militancy, which applies to trade union members as much as leaders, will be crucial if he is to succeed in his plan to reform and modernise public services.

And it will also have a big influence on the state of the UK economy.

With the government facing growing public sector budget deficits, it can ill afford major concessions on public sector pay which are not funded by productivity improvements.