Mr Martin is heading up a Commons inquiry into MPs' expenses
An investigation has begun into whether Commons Speaker Michael Martin has breached expenses rules.
The Parliamentary Commissioner for Standards is investigating reports Mr Martin's wife spent £4,000 of public money on taxis for shopping trips.
It follows a complaint from the Taxpayers' Alliance group which said Mr Martin should no longer chair a Commons inquiry into MPs' expenses.
John Lyon's office confirmed a preliminary inquiry had begun.
Mr Martin is currently heading up a wide-ranging inquiry into MPs' expenses, which was set up following revelations about Tory MP Derek Conway's use of expenses to employ his son.
But Mr Martin found himself at the centre of a story about his wife Mary's use of taxis and other stories about his family's use of air miles.
Mr Martin's aide, Mike Granatt, stepped down after saying he had unwittingly misled a journalist over a story that Mrs Martin had claimed more than £4,000 in taxi expenses since May 2004.
He had said the journeys were for shopping trips for official functions and that Mrs Martin had been accompanied by an official at all times, but he was later told by the Mail on Sunday that the official was in fact a housekeeper working for the speaker.
In a letter to the Taxpayers' Alliance, Mr Lyon said: "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."
The BBC understands the speaker's office has already responded to a request for information by Mr Lyon's office.
Mark Wallace, of the Taxpayers' Alliance, said: "Now the speaker is being investigated over his household expenses it would be completely inappropriate for him to remain in charge of the Parliamentary expenses system. He should hand over control of the review into MPs' expenses."
The Members' Estimate Committee, chaired by Mr Martin, is due to report back by the summer with its suggestions to overhaul the expenses system used by MPs.
The inquiry was set up after it emerged Mr Conway had employed his son as a researcher - while he was student in Newcastle - and made payments to him worth £40,000.
Mr Conway has since had the Conservative whip withdrawn, was suspended from Parliament for 10 days and ordered to repay £13,161.
A spokeswoman for the Commons authorities said Mr Martin's role with the Members' Estimate Committee was not in question.
"The work on the reform of allowances is continuing," she said, adding there would be no further comment until the commissioner's inquiry was finished.
Commons authorities are currently fighting at the High Court an Information Tribunal ruling that a detailed breakdown of 14 current and ex-MPs' claims under their second homes allowance should be published.
At the weekend it emerged that more than £700,000 of taxpayers' money had been spent on the speaker's official residence, inside the Palace of Westminster, since 2001.