The Speaker of the House of Commons, Michael Martin, faces mounting pressure after it emerged that he breached parliamentary guidelines by cashing in air miles earned on official business for flights for his family.
Air miles accrued on business trips should be used to reduce the cost to the taxpayer of future work-related flights, according to official advice to MPs.
But Mr Martin used his to knock £360 a head off the £3,090 total cost of business-class tickets for a return trip from Glasgow to London for his children and grandchildren over the New Year.
Mr Martin chairs the House of Commons Members Estimate Committee, which oversees MPs' expenses, and is also heading the parliamentary inquiry into MPs' expenses following the row over the former Tory MP Derek Conway's employment of his sons.
The Liberal Democrat MP Norman Baker, who has called for reform of Westminster perks, said: "This will further damage public confidence in the system of parliamentary expenses. A benefit which is accrued because of spending by the taxpayer should be returned to the taxpayer."
Last year, Mr Martin accrued £10,587 in parliamentary allowances to cover air travel on official business, much of it travelling between Westminster and his Glasgow constituency with his wife, who is entitled to travel with him at public expense on constituency and public business.
He also clocked up more than a million air miles, and used 54,000 of them to bring his son Paul, a member of the Scottish Parliament, and his daughter Mary Ann, who works as his constituency secretary, to London with their spouses and children. In all, there were seven in the party.
A spokesman for the Speaker, who earns £128,000 a year, said that Paul paid £309 and Mary Ann paid £230.50 towards the total flight costs.
Ministers are banned from using air miles for personal use but guidelines for MPs say that they should be used only for business. However, Commons officials have no means of enforcing that guidance.