There were renewed calls last night for the Speaker of the House of Commons Michael Martin to give up his role of overseeing MPs' expenses, after it was revealed that he is being investigated over allegations that he wrongly claimed public money to pay for taxis for his wife's shopping trips.
The inquiry into the Speaker's claims for £4,000 to fund his wife's use of taxis is another embarrassing blow to Mr Martin, who was criticised at the weekend for running up bills of more than £700,000 for the refurbishment of his official residence at Westminster.
The Parliamentary Commissioner for Standards, John Lyon, said he had launched a preliminary investigation after accepting that the spending on taxis "may not have been in accordance with the Code of Conduct for Members of Parliament".
Mr Lyon was asked to investigate a complaint from the TaxPayers' Alliance pressure group in February into whether Mr Martin had abused parliamentary expenses and allowances. Mr Lyon is obliged to examine all such complaints though he could have ruled that the complaint was unfounded. In a letter to the TaxPayers' Alliance, Mr Lyon wrote: "In essence, the part of your complaint I have accepted is that Mrs Martin's expenditure on taxis may not have been in accordance with the Code of Conduct for Members of Parliament and its associated rules."
He added: "I am inviting Mr Speaker to let me have his comments on your complaint. Once I receive his response, I shall consider how to proceed."
A spokeswoman for the Commissioner's office confirmed that a preliminary investigation had begun.
The Speaker's spokesman Mike Granatt resigned "for ethical reasons" in February after admitting he inadvertently misled a journalist over Mary Martin's taxi trips to buy food for "entertaining official visitors".
Mr Granatt had said Mrs Martin was accompanied by a Commons official, but in fact she was with her housekeeper.
The Speaker chairs the Commons Commission which is currently carrying out a comprehensive review of MPs' expenses in an attempt to clean up the image of sleaze associated with Parliament since the row over claims by the former Conservative MP Derek Conway.
Mark Wallace, the campaign director of the TaxPayers' Alliance, said it was now "completely inappropriate" for the Speaker to remain in charge of the Commons Commission which is due to deliver its anti-sleaze recommendations in the coming months.
"The Speaker is meant to oversee MPs' expenses and the propriety of their behaviour – an extremely important task at a time when the public are concerned at how their money is spent in Parliament," he said. "The controversial way Mr Martin has behaved by trying to keep MPs' expenses secret and running up questionable expenses bills has only added to that concern.
"Now the Speaker is being investigated over his household expenses it would be completely inappropriate from him to remain in charge of the parliamentary expenses system.
"He should hand over control of the review into MPs' expenses to someone independent and untainted by scandal."
The Speaker's allies said he had done nothing wrong, and that the bills for his official residence were caused because the building, part of the House of Commons overlooking the Thames, required expensive upkeep as a part of the national heritage.
* £4,000 claimed in expenses for taxis for shopping trips by the Speaker's wife Mary Martin. He said the trips were to buy food for official engagements.
* £700,000 of public money spent on the Commons Speaker's official residence at Westminster since 2001.
* More than £75,000 claimed in MPs' allowances for a second home, despite having no mortgage on the property in Glasgow.
* Publication of MPs' expenses pending a legal appeal delayed by Speaker as chairman of the Commons Commission.