Westminster's anti-sleaze committee has warned the Speaker, Michael Martin, that it may go ahead with its own investigation of MPs' expenses because of public perception that his own review is lacking independence.

The Committee on Standards in Public Life praises the way Holyrood brought in an outsider to conduct an equivalent review and insists that there would be more confidence in an independent study.

The Commons Members' Estimate Committee, chaired by Mr Martin, says it is carrying out a "root and branch" review of MPs' allowances. The Commons is to vote on changes to the system before MPs break-up for their summer recess in July but the Committee on Standards in Public Life indicated yesterday that it would not allow the issue to rest there unless the review reinstated trust in MPs' expenses.

Its chairman, Sir Christopher Kelly, who has recently spoken to Mr Martin, said: "It is essential that our elected representatives are seen to lead by example. The committee has publicly welcomed the recognition by the Members' Estimate Committee that one of the key requirements of any new system resulting from the current review is that it should command public respect.

"If the outcome of the current review fails to command public confidence then the committee is prepared to undertake its own independent review of the issues involved."

Sir Christopher's committee called for all expenses claims "wherever reasonably possible" to be backed up by receipts, which should be kept for up to three years.

MPs only have to produce receipts for items over £25, reduced from £250 following criticism of the allowances regime. In a list of principles the committee expects MPs to follow in their review, it stressed expenses should not be regarded as "entitlements" or a source of financial benefit.

"Members of Parliament should be entitled to be reimbursed for reasonable expenses incurred wholly, necessarily and exclusively in the performance of their parliamentary duties," said the watchdog committee, "but such expenses should never be regarded as a substitute for pay."

Many MPs say privately that they have historically accepted lower pay rises in return for more generous allowances.

Labour MP Chris Mullin, a member of the Commons Standards and Privileges Committee, recently said the £23,000 annual cap on their second homes allowance had become a "target to be aimed for".

The Committee on Standards in Public Life insisted MPs should be "as open and transparent as possible" with their expenditure of public money, subject only to security and data protection concerns, and said there should be no "double benefit" for MPs who are also government ministers.

A statement from the committee backed the Commons' steps but added: "The public credibility of the outcome of the review is, however, likely to be greater if it had been undertaken independently of the House of Commons, as with the recent review of parliamentary allowances in Scotland conducted by a panel chaired by Sir Alan Langlands.

"In the event that the outcome of the current review fails to command public confidence, we may wish to undertake such a review in any event."