The Speaker of the House of Commons has manoeuvred himself into an untenable position. He stands accused - not directly, but effectively - of misleading the public about taxi bills incurred by his wife and charged to the taxpayer; of using Air Miles acquired on official business to buy air tickets for his family; and of claiming expenses to help to cover the cost of a home in Glasgow on which he does not have a mortgage. At the same time it has fallen to the Speaker to order a review of alleged abuses of Westminster's complex and ill-defined expenses system by his fellow MPs. His approach has been widely and rightly criticised as lacking both urgency and independence. In these circumstances it is not just difficult to see how Michael Martin can honourably retain his post. It is impossible.

Last month revelations about the employment of family members at public expense by the Conservative MP Derek Conway threw into stark relief a hitherto inconclusive debate about MPs' earnings, allowances and expenses. Until then, two factors had deprived the debate of clarity or progress. First was the not unreasonable argument that MPs are underpaid relative to other professionals working long hours in stressful occupations in one of the world's most expensive cities. Generous allowances for travel and maintaining constituency as well as London homes were viewed not just as necessary for backbenchers to fulfil public roles but also as de facto top-ups on their £61,000 basic salary. Secondly, MPs had little interest in reforming a system with so much flexibility and so little oversight.

The Conway affair should have jolted the House of Commons out of such complacency. As it happens, the Standards and Privileges Committee has moved fast under Sir George Young to review MPs' expenses. But Mr Martin's preference was for it to move more slowly, and it was he who ensured that MPs would be policing themselves rather than submitting to outside scrutiny. Until the weekend it was possible to rationalise the Speaker's response in terms of his style. But Mike Granatt, his former spokesman, has now resigned in what appears to be genuine anger at having been misled by the Speaker's staff.

Mr Granatt had told The Times that the Speaker's wife's £4,280 taxi bill was all for official trips in official company. According to Mr Granatt, that company turns out to have been her cleaner, who is also a close friend, and the trips were largely personal. The Speaker is so far the subject of allegations only, but he is already guilty of giving the impression that his approach to reforming MPs' expenses has been guided as much by his own interests as by those of taxpayers.

It is entirely appropriate for Parliament to consider raising backbenchers' basic pay. As we have argued before, this would reduce their dependence on less than transparent expense claims, and ease the financial disincentive that dissuades too many high-flyers in other professions from entering public life. But Mr Martin is no backbencher. He earns £137,000 a year and lives in grace-and-favour splendour in the Palace of Westminster. Nor is his role merely that of exasperated referee in Commons debates. As chairman of the House of Commons Commission he is responsible for the honest management of British parliamentary democracy, whose reputation is battered enough without a tarnished Speaker. There is no formal mechanism for MPs to remove him, and he has said he will not yield to pressure. It is time for him to change his mind.