The Labour Party will learn on Friday whether it acted unlawfully in barring one of its affiliated trade unions from taking part in the contest to choose its London mayoral candidate.

The ruling could prove crucial in the party's internal election and could mean the difference between victory or narrow defeat for left-wing MP and mayoral hopeful Ken Livingstone.

Six London members of the Manufacturing Science and Finance (MSF) union have accused the party of acting unlawfully in barring the union from taking part in the electoral college vote for the mayoral candidate because its subscriptions to the party were paid late.

It is predicted that the majority of the MSF's 15,000 London region members would back Ken Livingstone, as would a substantial number from two other barred unions - the Rail Maritime and Transport (RMT), with 7,000 London members, and train drivers' union Aslef, with 3,000.

Last week London members of the Transport and General Workers' Union, which is taking part in the selection contest, voted for Mr Livingstone by a massive 85.8%.

His nearest rival, Glenda Jackson, won 7.3% while Frank Dobson, Prime Minister Tony Blair's preferred choice, came in last with a humiliating 6.9%.

Path could be cleared for Livingstone

If Aslef, the MSF and RMT unions are reinstated to the voting process, the close race between Mr Dobson and Mr Livingstone could result in the nomination of the former Greater London Council leader.

Laura Cox QC, for the six MSF members, told Mr Justice Jonathan Parker that their union was affiliated nationally and regionally to the Labour Party.

Under that affiliation, the six all paid a political levy to the union and they expected to be entitled to participate in the mayoral candidate selection process by voting in a ballot to decide who the MSF should support.

"We say they are being denied that right unlawfully and we seek a declaration to that effect," said Miss Cox.

The Labour Party, she said, argued that the union was not qualified to vote because it was not fully paid-up at the designated cut-off date of 31 December 1998.

But the fact was that, however late the payment was made, it was accepted by the party and formed the basis of a binding agreement entitling the union to participate in the party's activities.

Case 'misconceived'

Gavin Millar, counsel for the Labour Party, argued that the union members' case was "misconceived".

Labour's executive committee, he said, had quite properly decided last year to enfranchise only those organisations which were paid-up at the end of the last complete affiliation year - 1998.

There was nothing capricious or arbitrary in this, and the same procedure was adopted in the selection of Parliamentary candidates.

The six union members bringing the action are: Hugh McGrillen, former Labour general secretary Jim Mortimer, his wife Pat Mortimer, Sue Michie, Glen Sutherland and Ian Rez.