Trade unions must think again if they believe they can force MPs to vote on their behalf in parliament, the Commons speaker suggested on Wednesday.

Michael Martin's intervention followed John Prescott's resignation from the RMT after the union said it would only give cash to MPs who supported its demands for the complete renationalisation of the railways.

Whilst he rejected demands for the RMT union to be referred to the Commons standards and privileges committee, he issued a coded warning to MPs and trade unions.

Speaking after being called upon to adjudicate on whether the RMT's actions had breached the rules of the Commons, Martin said it was wrong for any MP to enter into a "contractual agreement with an outside body" which controlled or limited their independence.

"The House expects at all times that Members will take the greatest care to ensure that their relationships with all outside bodies will be in strict conformity with our rules," he said.

Brian Mawhinney, the former Conservative Party chairman, said the speaker's intervention amounted to a warning to trade unions.

"Would we be right in assuming that your statement is a warning to outside bodies to stay well clear of any behaviour which might even raise the suspicion of a breach of privilege," he said.

Speaker Martin said external bodies "must interpret my ruling as they see fit".