Parliament's standards watchdog is considering a complaint over Commons Speaker Michael Martin's use of air miles for flights for his family.
Mr Martin is heading an inquiry into MPs' expenses.
Mr Martin is accused of flying members of his family business-class from Glasgow to London for a New Year break.
The maximum official price of the return flights was about £3,000 but the family is thought to have saved about £360 a head by using the air miles.
A spokesman said his son paid £309 and his daughter £230 towards the costs.
The complaint against Mr Martin was brought by Michael Barnbrook, a retired policeman and former candidate for the UK Independence Party, who is now a member of the British National Party.
A spokeswoman for John Lyon, the Parliamentary Commissioner for Standards, said: "I can confirm that he has received a complaint on that subject about the Speaker and he is considering it."
However, she said Mr Lyon would "not be giving a running commentary" on the issue.
But, she confirmed that the commissioner is also considering a further complaint from Mr Barnbrook against husband and wife Tory MPs Sir Nicholas and Ann Winterton.
The couple have defended their use of expenses for a flat, even though they had paid the mortgage.
Mr Martin, chairs the Commons Members Estimates Committee, which oversees MPs' expenses.
He is also heading a "root and branch" parliamentary inquiry into all aspects of MPs' expenses, in the wake of the furore over ex-Tory MP Derek Conway's employment of his sons.
A spokesman for Mr Martin and the Commons authorities said the Speaker had amassed more than 800,000 air miles on official business but had never before used them.
They were used on this occasion because he was detained at Westminster hosting a Commonwealth Speakers' conference and did not wish to be away from his family over the New Year period, the spokesman said.
The contribution towards the cost of the tickets was said to be roughly the equivalent to airport charges and taxes.
Mr Martin's daughter Mary Ann flew with her husband and son from Glasgow to London on 28 December, returning on 6 January. His son Paul travelled with his wife and two daughters on 30 December, returning on 2 January.
The spokesman said there were no rules about the use of air miles and the issue had never been discussed by the Commons Estimates' Committee, which oversees such matters.
If asked, officials tell MPs to use air miles against future official flights if possible, but this is not binding advice and is not volunteered unless requested.
Last year the Speaker clocked up £10,587 in parliamentary allowances to cover air travel on official business. Much of this was for trips with his wife Mary between Westminster and their Scottish home.
Liberal Democrat MP Norman Baker, a staunch supporter of a crackdown on 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."