Bob Crow said it was not in his members' interests to be affiliated

The Rail, Maritime and Transport (RMT) said the decision had been taken after consultation with branches in Scotland.

The rail union, which represents 7,000 workers in Scotland, threw its support behind the Socialists. It was expelled from the Labour Party in 2004.

Leaders insisted the union had not decided to affiliate with Tommy Sheridan's new Solidarity movement.

Political developments

RMT general secretary Bob Crow said: "It gives no pleasure to have to take this decision, which has been made in close consultation with our Scottish committee and branches.

"However, it is clear that there are elements within the SSP that have destabilised the organisation, that the atmosphere within it is no longer conducive to comradeship, and that it is no longer in our members' interests to remain affiliated.

"We will of course be monitoring political developments and the question of any possible future affiliation will be decided, as allowed under rule, in line with the union's aims and objects and the wishes of the membership."

A union spokesman said about �10,000 in fees and donations were made to the party last year, but that figure can vary year on year.

The move follows the high-profile departure of Mr Sheridan, the party's former leader, in August amid political infighting.

We share a mutual loathing for New Labour and all it represents Colin Fox SSP leader

The SSP co-founder won a �200,000 defamation case against the News of the World only to later split from the SSP amid public acrimony and found his own party, Solidarity.

SSP leader Colin Fox said the RMT decision was not entirely unexpected and that the union had not given the party the opportunity to state its case.

"The fact is that the RMT did not support the party's position regarding Tommy Sheridan's court case," he added.

"I believe that we were entirely right and that subsequent events have entirely vindicated us on this.

"I would hope that everyone would recognise that all the reasons the RMT gave for affiliating remain entirely valid; we share a mutual loathing for New Labour and all it represents, their attacks on working people, support for illegal bloody wars and anti-union laws are anathema to both of us."

The union, which has about 75,000 members, was expelled from the UK Labour Party in February 2004 after Scottish members decided to affiliate with the SSP.