We have to make decisions about how best to campaign on behalf of our members in the postal industry

Billy Hayes

It also came as the leader of the transport union RMT, Bob Crow, stepped up calls for Labour MPs to do more to represent the views of his members.

Labour general secretary Mr Triesman said, in an interview with The Times, that to be swayed by financial concerns would be "unethical".

"You cannot have an ethical party or an ethical government if people believe that their donations can buy them something," Mr Triesman told the newspaper.

"We know that people are saying this or that union will cut the amount it gives unless the party delivers the following things....," he said.

"Unions are saying in an inverse way that money is associated with policies."

Strike considered

But speaking on BBC Radio 4's Today programme on Friday, Mr Crow, said that his members wanted to get better respresentation of their views in Parliament.

He said the majority of his members opposed the privatisation of London Underground, and questioned why they would want to support MPs who supported that move.

The implicit threat that the RMT might cut its funding came as the CWU slashes �500,000 - a third - of its donations over the next three years.

General secretary Billy Hayes said: "Like any other organisations, we have a limited budget.

We value the institutional link with the Labour Party. Remember, the last time this was broken was at the instigation of the right-wing government of Stanley Baldwin in 1927

Billy Hayes CWU

"We have to make decisions about how best to campaign on behalf of our members in the postal industry, in the Girobank and in telecommunications."

Currently the CWU is threatening a strike over plans by the Post Office - now called Consignia - over the job cuts.

Mr Hayes said the money that was kept back from the Labour Party would be used to fund an anti-privatisation campaign.

"We are fighting to end the deregulation, and de facto privatisation, of the postal industry.

"We are struggling to assure a future for our financial services members, especially in the Girobank; and we are campaigning for a broadband Britain in which BT has a leading role. All these things cost money."

Asked whether the decision to deprive Labour of some of its usual donation was purely symbolic, Mr Hayes said he hoped the party leadership would take notice.

"We value the institutional link with the Labour Party. Remember, the last time this was broken was at the instigation of the right-wing government of Stanley Baldwin in 1927.

"We have no desire to return to those dark days."