Mike Granatt said he had quit for "ethical reasons" after "unwittingly" misleading a journalist over Mr Martin's wife's expenses.

He said it was impossible to continue after he briefed wrong information relating to Mary Martin's taxpayer-funded taxi journeys.

"I have stepped down from my post for ethical reasons, because I misled a journalist unwittingly," he said.

Mr Granatt had briefed that Mrs Martin was accompanied by a Commons administrative official on taxi trips for which she claimed more than £4,000 in expenses.

He admitted that that had not been the case and Mrs Martin had in fact been joined by a housekeeper. Officials who knew the truth of the matter had previously failed to point this out to him, he said.

He was therefore standing down from his position, officially media adviser to the House of Commons Commission, the management committee which Mr Martin chairs.

"It is core to the ethical code by which I and my company operate that I tell the truth, and that I am given the truth to tell," Mr Granatt said. "However, I learned on Friday that I had been led to mislead journalists over material facts in a story concerning the Speaker's household and the use of taxis. I have expressed my regrets to the journalist who brought this to my attention, and I offer them to anyone else who was similarly misled. I wish to make it clear that this arose through no fault of Mr Speaker."

Mr Granatt's resignation is another blow to the Speaker, who is already under pressure over his use of parliamentary allowances. Last week it was revealed he had used air miles earned on official business to pay for flights for family members.

A group of MPs is said to be agitating for him to move on shortly after years of concern about his performance.