Speaker's future in jeopardy as aide quits

Michael Martin, the beleaguered House of Commons Speaker, suffered a fresh, and potentially lethal, political blow last night after a senior aide quit, saying he had been misinformed by Mr Martin's staff during a recent row over taxi bills for his wife's shopping trips.

Mike Granatt, Mr Martin's official spokesman, announced he had resigned after admitting he had "unwittingly" deceived journalists over revelations that Mary Martin had run up a £4,280.20 taxi bill in the past four years.

Mr Granatt, a former government press chief who is now a partner at City PR firm Luther Pendragon, had insisted that Mrs Martin was "always accompanied by an official from the Speaker's office in this task". That suggestion had bolstered the defence that the journeys were on official business.

But Mr Granatt has now confirmed that Mrs Martin had in fact been accompanied by a housekeeper who is also believed to be a close friend. It was reported last night that the journeys had not been food shopping for official functions but for informal occasions.

"It is core to the ethical code by which I and my company operate that I tell the truth and I am given the truth to tell," Mr Granatt said in a damaging statement announcing his departure. "I learned on Friday that I had been led to mislead journalists over material facts of a story concerning the Speaker's household's use of taxis. I have expressed my regret to the journalist."

Mr Granatt insisted that he had been misled by staff in Mr Martin's office, and not by the Speaker himself. But the loss of his experienced spokesman increases pressure on Mr Martin as he attempts to head off mounting discontent over his performance in the Commons and revelations about his use of benefits associated with the post.

Parliament's standards watchdog is already investigating a complaint from an MP that the Speaker used air miles accrued on official business to help get cheap business-class flights for members of his family over the Christmas period.

Mr Martin also faced new questions over his expenses last night after it emerged he has no mortgage on his luxury Glasgow home – even though he has claimed more than £75,000 in MPs' expenses for the property over the past six years. The couple's main residence is a grace-and-favour suite of apartments within the Houses of Parliament.

A growing number of senior figures within and outside Parliament are expressing deep concerns over Mr Martin's ability to restore the reputation of MPs following a series of damaging revelations about their lavish expenses and allowances. He is supposed to head a "root and branch" parliamentary inquiry into all aspects of MPs' expenses, following the furore over Tory MP Derek Conway's employment of his sons.

It was reported yesterday that four senior MPs are being canvassed to see if they would be interested in becoming Speaker.

Anti-sleaze campaigner Martin Bell was one of the first to call for Mr Martin's resignation last night. "If you live free in a grace-and-favour home, you shouldn't need public help to run your second home," Mr Bell, former independent MP for Tatton, said. "The Speaker should announce his retirement; the Commons is now in such disrepute we need a new Speaker who can help clean it up."

The prestigious post of Speaker comes with a £130,347 salary and the Westminster residence. But once they have elected a Speaker, MPs are powerless to get rid of him.

Friends of the Speaker insist that he is determined to stay on until after the next general election, claiming he is the victim of snobbery and discrimination.

Mr Martin was a controversial choice as Speaker in 2000 and his record since, including complaints of partisanship from Tory MPs, has made him unpopular among opponents and colleagues alike. But the switch in focus to recent allegations of impropriety in connection with parliamentary expenses has put his financial affairs in the spotlight and increased the pressure on him. Mr Granatt's dramatic move has brought further scrutiny of the Martins' financial affairs, and might hasten the Speaker's eventual departure.

A growing number of MPs were privately expressing the hope last night that the series of revelations will force Mr Martin's hand, or encourage the Prime Minister to exert subtle influence and persuade him to stand down early.

A spokesman for the TaxPayers' Alliance said: "People have precious little faith in politicians as it is, without such an iconic figure as the Speaker being seen to have his snout in the trough."

Speaker's corner: The trials of Mr Martin

November, 2000: Claims MPs' betting ring won £50,000 on Mr Martin's appointment the previous month. One MP censured.

November, 2006: Causes uproar during PMQs by striking down David Cameron's challenge to Tony Blair over future Labour leadership.

October, 2007: He is criticised for £20,000 of taxpayers' money paid to law firm Carter-Ruck, to challenge negative press.

December, 2007: Emerges Mrs Martin spent more than £4,000 on taxis in four years. Spokesman Mike Granatt explains she was "always accompanied by an official from the Speaker's office".

February, 2008: Emerges Martin used air miles accumulated on official business on relatives' travel.

February, 2008: Granatt resigns for "ethical reasons", saying Speaker's staff caused him to mislead journalist over Mrs Martin's expenses.