Michael Martin, the Commons Speaker, was fighting to save his political career last night after his senior media adviser quit, claiming he had been misled over taxpayer-funded shopping trips.

Mike Granatt, the Speaker’s spin doctor, resigned after it emerged he had been kept in the dark about £4,280 claimed on expenses for taxis used by Martin’s wife Mary.

Granatt becomes the latest in a series of officials who have left the Speaker’s office in controversial circumstances.

Martin also faces new criticism over his expenses claims after it emerged that he has claimed thousands of pounds to cover costs for his home in Glasgow when the property does not have a mortgage.

He received £17,166 in expenses for the property last year. He is also using his Scottish home as an office – a practice from which MPs are discouraged – claiming £7,595 last year for running costs.

Yesterday the pressure on Martin to announce his retirement intensified after one MP described Granatt’s resignation as a “significant and worrying development”.

Granatt, a former director of the government information service and a media consultant, telephoned the Speaker yesterday to tell him his decision.

“It is core to the ethical code by which I and my company operate that I tell the truth,” said Granatt. “I have found it no longer possible to work as the media adviser to the House of Commons Commission [chaired by Martin], and I have informed Mr Speaker that I am stepping down immediately.”

The row surrounds information released two months ago about the expenses of the Speaker’s household, which showed Mary Martin had claimed more than £1,000 a year for taxis since May 2004. At the time, Granatt told reporters the trips were “entirely in connection with household expenditure that supports the Speaker’s duties”, adding that she needed to take taxis to shop for food for official functions.

Granatt also insisted that the Speaker’s wife had always been accompanied by an “administrative official”. However, after media inquiries last week, the spin doctor discovered this “official” was the Speaker’s housekeeper.

In his resignation statement Granatt said: “In the statement I gave to journalists, the obvious implication was that Mrs Martin was accompanied by an administrative official. It was in fact the housekeeper who provides domestic support to Speaker’s House.

“I had been led to mislead journalists over material facts in a story concerning the Speaker’s household and the use of taxis.”

Granatt said it was neither the Speaker nor his wife who had provided the misleading information. “The statement was approved by people who knew the facts,” he said. “No one brought this discrepancy to my attention. This arose through no fault of Mr Speaker.”

The incident will raise fresh questions about the culture in the Speaker’s office. Last week he was criticised for using Air-miles earned on official business to pay for flights for family members.

Martin has also been attacked for his slow response to the scandal over Derek Conway’s employment of his sons on Commons expenses.

Norman Baker, the Liberal Democrat MP who campaigns against parliamentary sleaze, called for a formal investigation into Granatt’s resignation. “This is a significant and worrying development. We need a full explanation of what happened here,” he said.

Granatt, 57, now a partner in public relations firm Luther Pendragon, was director-general of the government information service under Tony Blair and fought to maintain the impartiality of the service amid constant battles with Alastair Campbell.

He is the latest member of staff to have quit the Speaker’s office. Martin parted company with Charlotte Every, his diary secretary, a year after taking office. Two Speaker’s secretaries have also departed: Sir Nicholas Bevan, who retired early amid rumours of rows with Martin, and Roger Daw, who left after 18 months.

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