Prime Minister Gordon Brown's new sleaze adviser has told MPs he will not be anybody's "footstool or patsy".
Sir Philip was grilled by the public administration committee
Sir Philip Mawer told MPs he was personally recruited by Mr Brown to police ministerial standards and guard against conflicts of interests.
But he said the advice he gave would be independent and he would stand up to the prime minister if necessary.
He also proposed issuing "yellow cards" to ministers facing sleaze allegations before they are forced to quit.
In his former role as Parliamentary Standards Commissioner, Sir Philip investigated complaints against MPs.
He was appointed to the new role of the prime minister's adviser on ministerial interests by Mr Brown - although it remains the case that it is up to the prime minister to decide whether the ministerial code has been broken.
Sir Philip told the public administration committee he is employed as a consultant to the prime minister rather than a member of the civil service.
But he said: "That does not mean I am on the coat tails of the prime minister."
He said he would offer independent advice and challenge the prime minister when he believed an investigation was needed.
"I can assure you that I am not going to sit there supine. If I believe that something on the basis of the facts available to me requires investigation I shall make that view known to the Cabinet Secretary and the Prime Minister.
"So I haven't been brought into this job just to be somebody's footstool or patsy."
Asked about one of his predecessors as Parliamentary Standards Commissioner, Elizabeth Filkin, who claimed her work was frustrated by a whispering campaign against her by senior politicians, Sir Philip said he would be prepared to take the "ultimate sanction" of quitting.
"If I believe the conditions that are being placed on me mean I am not able to do the job properly I will have no hesitation in saying so," he told MPs.
Sir Philip revealed he had known Mr Brown since their university days in Edinburgh in the early 1970s, when he was president of the student council and the future prime minister edited the student newspaper.
But he said Mr Brown was more of an "acquaintance" than a friend.
He also told MPs he was considering a system of "yellow cards" for ministers facing misconduct allegations should be given a "yellow card" to prevent them being unfairly hounded out of office by the press, who he said were sometimes guilty of "dishonest" reporting.
"The press don't get into these matters without some cause for doing so - so politicians have to look to their own behaviour and not give reason for the press to become interested in the first place.
"But the press can, in their treatment of these matters, be disproportionate and, frankly, dishonest, on occasion."
He suggested "very minor" breaches of the ministerial code could be punished by an apology to Parliament, although it would be up to the prime minister to decide what the tariff of penalties for errant ministers should be.
But he added: "I do think it is in everybody's interests to try to work out something falling short of the red card on every occasion.
"Because the trouble with that is that it simply feeds the public view that all these people are guilty and needs to be turfed out at the earliest opportunity."
The idea of a "yellow card" system was first proposed last year by Sir Alistair Graham, former chairman of the Committee for Standards in Public Life, as part of a broader call to improve ministerial standards.
Sir Philip was also questioned on whether ministers should be banned from taking jobs in the private sector when they quit government.
He said he would be monitoring ministers' behaviour to see if they were becoming too close to business interests or "trimming" their decisions with an eye to future employment.
But he rejected a complete ban as it would be difficult to police and would leave ministers with few career options after politics.
"You would have to have some arrangement in place that would allow you to retrain and re-equip those minsters. 'Fresh Start for Ministers' perhaps," he said.