Pressure is growing on Michael Martin to stand down as Speaker of the Commons after one of his aides quit for misleading The Times, and after a second complaint about his conduct was submitted to the parliamentary watchdog.

Mike Granatt, who was Mr Martin’s spokesman until Friday, resigned claiming that someone in the Speaker’s office had lied to him – either deliberately or accidentally – over the use of taxis by the Speaker’s wife, Mary Martin.

The Parliamentary Commissioner for Standards has now been asked to investigate whether more than £4,000 spent on taxis for Mrs Martin’s shopping trips amounted to an inappropriate use of public money.

As MPs continued to refrain from criticising the Speaker, Martin Bell, a former independent MP, claimed that there was a “wall of silence”. He said: “MPs can talk about anything they like, inside or outside the House, except their views about the Speaker. They do not speak up. We know there is widespread disquiet on both sides of the House and no one dare speak up.”

David Davis, the Shadow Home Secretary, became the first MP to acknowledge the difficulties that Mr Martin was facing. He told The Andrew Marr Show on BBC One: “Clearly, he has got problems. The House of Commons needs to be much more transparent.”

Several MPs contacted yesterday by The Times criticised Mr Martin’s conduct privately, saying they hoped that he would stand down at the next general election, but they refrained from speaking publicly. However, many Labour MPs now see this as a witch-hunt and have rallied round the Speaker. The Tories fear that if Mr Martin stands down now and a Conservative is elected, Labour will insist on reverting to one of their own after a general election.

Mr Martin’s problems stem from December when The Times revealed his wife’s use of taxis. Mr Granatt, who represented the Speaker, said at the time: “She goes shopping for food and so on for entertaining official visitors.” He added: “She is always accompanied by an official from the Speaker’s office in this task.”

Yesterday The Mail on Sunday revealed that the “official” was Gloria Hawkes, the Martins’ housekeeper, who is also a friend of Mrs Martin, and that the Commons’ catering services provide all the food for official functions. This prompted Mr Granatt, a well-known figure in Whitehall departments, to resign for “ethical reasons”.

In a statement he said: “I have found it no longer possible to work as the media adviser to the House of Commons Commission and I have informed Mr Speaker that I am stepping down immediately.”

“I learnt 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 want to make it clear this arose through no fault of the Speaker.

“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. The statement was approved by people who knew the facts. No one brought this discrepancy to my attention.”

Mr Granatt would not disclose yesterday whom he had spoken to in the Speaker’s office or who had approved the original statement given to The Times.

Yesterday a complaint was submitted to John Lyon, the Parliamentary Commissioner for Standards, by Mark Wallace of the TaxPayers’ Alliance.

He asked the commissioner to investigate whether the £4,000 was claimed improperly for the unofficial shopping trips, whether the money should be paid back, why the public was misled and when the Speaker was aware of this. He also questioned whether the Speaker should continue to oversee the investigation into expenses after the Derek Conway scandal.

Mr Wallace said: “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 allow us to know the truth once and for all.”

Mr Martin also faces new criticism over his expenses after it came to light 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 for the property last year. He is also using his Scottish home as an office – a practice that is discouraged – claiming £7,595 last year for running costs.

Sir Alistair Graham, the former sleaze watchdog, called on Mr Martin to hand over the entire investigation to an independent body. “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 that he hasn’t broken any rules,” Sir Alistair told the BBC.

Sir Alistair said that the Committee on Standards in Public Life, which he chaired until last year, should be given charge of the review that is being undertaken by the Members Estimate Committee.

Labour friends of Mr Martin rallied to his defence yesterday.

Lord Foulkes of Cumnock, a leading Scottish Labour politician, said that the stories about Mr Martin were “a lot of unsubstantiated smear and innuendo” and part of a battle drawn out along class lines. He said that the campaign against Mr Martin was started years ago by “people who went to private schools and Oxbridge who didn’t like someone from a working-class background in Glasgow getting into the highest office in the land”.

Tony McNulty, the Home Office Minister, agreed that greater transparency was necessary but said that allegations about MPs’ expenses were being thrown around “like confetti”.

Margaret Beckett, the former Foreign Secretary, told Andrew Marr that there had been “a whole string of nasty stories” about the Speaker. She added: “Clearly someone is out to get him. Whether any of it is valid, I cannot judge.”

The Speaker’s bill

£137,000 Salary; he also gets a grace-and-favour apartment

£17,346 Expenses he claimed for a second home last year

£7,595 Expenses he claimed last year for running costs for using his Scottish home as an office, a practice from which MPs are discouraged

£50,000 Amount of public money he spent on air travel for his wife, Mary. She also spent more than £4,000 on taxis

£21,500 Amount of public money spent on legal challenges by Mr Martin’s office to newspapers, including The Times

£3,000 Value of Air Miles used by Mr Martin to fund flights between Scotland and London for his family. Guidelines stipulate that Air Miles should be used to reduce the cost of travel paid for by the taxpayer

£15,201 He also claimed £3,138 in car allowances, £10,587 in flights and £1,476 on stationery and postage

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