A Labour MP has urged Whitehall's sleaze watchdog to launch an "independent" probe into MPs expenses.
Sir Christopher Kelly is a former senior civil servant
Tony Wright says he fears Sir Christopher Kelly, chair of the Committee on Standards in Public Life, may be under pressure not to do so.
Gordon Brown has ordered Labour MPs to give details of relatives they employ. David Cameron also wants his senior MPs to outline their expenditure.
It comes after Tory MP Derek Conway was suspended from the Commons.
In a letter to Sir Christopher, Dr Wright said he would be right to resign his post after two months rather than cave in to any pressure.
He said it was "not acceptable in terms of public trust" that the internal Commons Members Estimates Committee, chaired by Speaker Michael Martin and made up of senior MPs, should be the only body to look into the situation.
Dr Wright, chair of the Commons public administration committee, said that the current allowances system was "wide open to abuse on a number of fronts".
He claimed opportunities for politicians to use public cash for private gain amounted to corruption and should be rooted out to restore public trust in Parliament.
He wrote: "If this is not something for your committee to respond to, in view of the reasons for its existence, I do not know what is...
"We urgently need an inquiry that is independent and authoritative. Only your committee can undertake this, and the restoration of public trust demands it.
"It is simply not acceptable, in terms of public trust, for an internal Commons committee to be the only body to examine this issue.
"Nor is it only a question of transparency and better reporting. The whole allowance system needs examining, and justifying, along with arrangements for audit and scrutiny.
"The fact is that the current system is wide open to abuse on a number of fronts."
He added: "You told me that you had only accepted the chairmanship of the committee on the basis that it would not be prevented from examining any area of concern.
"I suspect that there is pressure on you currently not to get involved. I urge you to resist such pressure and to insist on an immediate inquiry being launched by the committee. This is precisely what it was set up for.
"In my view you would be fully entitled to say that you were unable to continue in your role if you were in any way prevented from undertaking this inquiry now."
Last week Derek Conway had the Tory whip withdrawn and was suspended from the Commons for 10 days over payments to his son Freddie out of his MP's allowance.
And questions have been raised about the system of allowances and expenses which last year saw MPs mount up a total bill of £87m - an average of £137,000 each - on secretaries, assistants, housing costs, travel and stationery.
Ex-prime minister John Major set up the Committee on Standards in Public Life in 1994 as "an ethical workshop called in to do running repairs" to public life.
Its remit, which does not focus on alleged misdemeanours by individuals, is described by the committee as a "useful check on current standards and the effectiveness, or otherwise of the arrangements in place to ensure the highest standards of propriety in public life".