Following John Prescott's resignation from the RMT, Robin Cook has warned the union that MPs cannot be "bought".

The leader of the Commons said that while no offence had been committed, members of the House must remain "free representatives of our constituents".

The intervention came after John Prescott announced he had resigned his membership of the RMT in protest at the union's opposition to the government's policy agenda.

The union severed cash links with Prescott after RMT insisted that MPs sign an oath of loyalty in return for cash donations.

The Rail Maritime and Transport Union decided to halt its funding of 13 Labour constituency parties - including the deputy prime minister's Hull East seat and Robin Cook's Livingston seat.

Liberal Democrat spokesman Paul Tyler said there were implications for parliamentary privilege in the RMT's attempt to impose conditions on its donations to MPs.

He welcomed Robin Cooks decision not to accept the RMT's offer and the deputy prime minister's resignation from the union.

"It is not just unacceptable, it is a potential contempt of this House, that any union, any outside organisation should seek to influence the way in which the MP in parliament votes and speaks," said Tyler.

He noted that in 1974, when his union had sought to influence his vote, the Speaker had forced the union to apologise.

It was a "very serious issue indeed" said Tyler.

Cook said that his long relationship with the RMT "had been useful to both of us" especially in fighting elections.

"Whilst I welcome support in fighting those elections, from anybody who broadly supports Labour's values and wishes work for a Labour victory, I will not be bought for any particular agenda," he said.

"It is on that basis that I declined to sign up to an oath of loyalty. I know every other single RMT MP in the House has also similarly declined."

Cook said it was not for him to say whether the union was guilty of contempt.

"There first of all has to be an offence. No offence has occurred in this case, as all of those involved have made it quite clear we are not going to sign up to an oath of loyalty to anybody outside this House," he added.

"And that has to be a very important powerful principle of membership of this House, that we come here as free representatives of our constituents, with, of course, our political mandate of the party, and we are here to make sure that we exercise our best judgement on behalf of our constituents in line with the mandate that we receive.

"We are not here to act on behalf of any other specific or particular vested interest or agency."

On Tuesday the union decided to offer its funds, worth £44,000 to each constituency, to more left-wing MPs.

RMT members also backed a measure to cut donations to the Labour Party itself from £110,000 to £20,000.

The union said its move was prompted by Labour's failure to renationalise the railways.

Bob Crow, general secretary of the RMT, said Labour had betrayed working people.

But Prescott said it was "unacceptable" for the union to tell MPs how to vote.

"As a long-standing MP and trade unionist it is unacceptable that my trade union, the RMT, should dictate how a Member of Parliament should vote," he said in a statement.

"The recent requirement of the new RMT leadership for a loyalty test is designed to do just that but this is not only totally unacceptable to me, it is contrary to my obligation to my constituents, to the rules of the Parliamentary Labour Party and would constitute a breach of parliamentary privilege.

"The endorsement of this policy by the RMT conference on such a fundamental principle leaves me feeling that I have no alternative but to resign my membership of the RMT, " he said.

"I have always championed the rights of working people by choice and conviction but never by coercion and will continue to do so but I am convinced that this kind of policy will undermine the historic and important relationship between the trade unions and the Labour Party and will be detrimental to trade union members," he added.

Prescott said his resignation was a "personal and very sad day for me". He first joined the union as a 17-year-old merchant seaman.

As transport secretary before last June's general election, Prescott had been responsible for decisions on the future of the railways.

He has defended his decision at the time not to renationalise Railtrack, saying the money was better spent investing in infrastructure than compensating shareholders.