In the face of more expenses revelations, how long can Speaker Michael Martin keep hanging on?

There were calls for him to face a standards inquiry over his wife claiming £4,000 in taxi bills as official expenses.

One of his key aides resigned, claiming he had been lied to about Mary Martin's tax-payer-funded shopping trips.

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Many MPs fear that, after nearly eight years in office, 62-year-old Mr Martin no longer has credibility as the public face of the Commons at a time when the behaviour of MPs is under scrutiny.

The latest embarrassment for the £138,000-a-year Speaker came with the departure of his spokesman and media adviser Mike Granatt.

It was Mr Granatt who said last December that Mrs Martin had racked up taxi bills of £4,280.17 since 2004 "entirely in connection with household expenditure that supports the Speaker's duties", and had been accompanied by a Commons official.

But at the weekend, he had to admit that the trips were linked to "informal" entertaining, and that the "official" with Mrs Martin's was in fact her housekeeper Gloria Hawkes, a friend and confidante.

Mr Granatt said he was quitting his £25,000-a-year part-time job with the Speaker for ethical reasons after discovering he had given journalists misleading information.

He issued a damning statement which appeared to question Mrs Martin's honesty while stressing her husband was not at fault.

Mr Granatt said "people who knew the facts" had approved his December statement without informing him of the discrepancy.

The new twist prompted the campaign group Taxpayers' Alliance to request a formal inquiry by the Parliamentary Commissioner for Standards, John Lyon.

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Mr Lyon is already considering an investigation into claims that Mr Martin misused air miles earned on official business to fly relatives to London for a family gathering.

There is also concern over his use of the second home allowance after it emerged he has no mortgage on a property for which he has claimed more than £75,000 to maintain.

Labour grandees are waging a concerted campaign to prop up the Speaker, arguing that he has not broken any rules, but there are growing doubts over his future.

His decision to put a group of cronies in charge of reviewing the system of allowances to MPs in the wake of the Derek Conway scandal dismayed many MPs who believe more radical action is needed.

The Tories said Mr Martin "has got a problem" but refused to go further, fearing they could suffer a Labour backlash if they are seen to be hounding him from office.

Anti-sleaze campaigner Martin Bell said Mr Martin was being protected by the convention that MPs do not openly criticise the Commons Speaker.

The former independent MP said: "He is protected by a wall of silence, actually, because MPs can talk about anything they like, inside or outside the House, except their views about the Speaker.

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"We know there is widespread disquiet on both sides of the House but no one dare speak up."

He was backed by former sleaze watchdog Sir Alistair Graham, who urged Mr Martin to pass responsibility for a review of MPs' expenses to an independent body.

He said: "It's unfortunate and really does undermine the credibility of this review that he himself has become part of this story, although it's clear he hasn't broken any rules. But it does suggest the rules are inappropriate and need radically reforming."

Sir Alistair added: "The scope for reform is pretty enormous but unfortunately now the Speaker and the committee the Speaker chairs is not the body to carry out that reform."

Mark Wallace, Taxpayers' Alliance campaign director, said: "The public are extremely concerned that some politicians may be taking advantage of the money provided to them to support the democratic process.

"If an iconic figure like the Speaker has done so then people will be appalled, especially as he has responsibility for making sure other MPs follow the rules. An investigation will let us know the truth once and for all."

Senior Labour figures rallied to defend Mr Martin, the MP for Glasgow North-East. Former Home Secretary David Blunkett said he was the victim of a "witch-hunt" while ex-Foreign Secretary Margaret Beckett said: "Clearly someone is out to get him. Whether any of it is valid, I can't judge."

Labour peer Lord Foulkes branded the allegations against the Speaker "unsubstantiated smear and innuendo".

He claimed there was a campaign by people who did not want a working-class former sheet metal worker "getting into the highest office in the land".

Even though Tory MPs overwhelmingly want to get rid of Mr Martin, David Cameron does not want to see him quit before the next election.

He fears Labour MPs would use their majority to retaliate by electing another of their own as Speaker, the third in a row.

Neither Mr Cameron nor Gordon Brown is prepared to pressure Mr Martin to quit, making a Commons coup to oust him highly unlikely.

Instead, MPs speculated that he may be persuaded to announce, possibly once the current row cools, that he will stand down at the next election.