Culture Secretary Tessa Jowell is facing continuing pressure over her husband's financial affairs, despite being cleared of any wrongdoing.
The prime minister has given Ms Jowell his full support
Tony Blair said she had not been "in breach" of the ministers' code of conduct as her husband did not tell her about a £344,000 gift he had received.
Cabinet Secretary Sir Gus O'Donnell reported on the matter on Thursday.
Labour's Peter Kilfoyle said Ms Jowell should consider quitting. Tory Nigel Evans called the report a "whitewash".
The BBC's political editor Nick Robinson said some people were questioning the credibility of the ministers' code, when a decision could be made by the prime minister concerning a "political ally".
While friends and supporters say Ms Jowell has cleared up the matter about the gift, her enemies say "not good enough", he added.
However, Chancellor Gordon Brown has offered Ms Jowell his backing.
In his report, Sir Gus said the culture secretary accepted her husband David Mills should have told her about the £344,000 gift.
She would then have told her top civil servant about the gift - so complying with the code - his report said.
Conservative MP Nigel Evans, whose request for a Commons debate on the ministerial code was rejected on Thursday, told BBC Radio 4's PM programme he was "absolutely staggered" by the report.
"The fact is that she knew what the code said. She should have told her husband what the code said. He should have told her about the substantial gift and she should have declared it," he said.
"The ministerial code is in tatters unless she goes".
Conservative MP Sir Nicholas Winterton told BBC One's Question Time: "The prime minister has to stand by the decision which he makes in respect of a leading friend and cabinet minister."
The Liberal Democrat's Sarah Teather told the programme: "Is it credible that she did not know (about the gift)? Well it probably is. It is possible she did not know."
Ms Jowell, the minister in charge of the winning London Olympic bid, issued a statement on Thursday saying: "I have always discharged my responsibilities under the ministerial code in good faith."
Italian prosecutors have been examining claims the £344,000 payment was made to Mr Mills, an international lawyer, in return for helpful testimony in a corruption probe into Italian premier and media magnate Silvio Berlusconi in 1997.
Mr Mills denied that the money came from Mr Berlusconi, saying it was from another client. Both he and Ms Jowell have denied any wrongdoing.
Ms Jowell was drawn into the affair after it emerged that she had co-signed a £408,000 loan taken out against the value of their house, which was paid off just weeks later, apparently using the Italian money.
She said it was not "unusual, improper or illegal" for her to take out a mortgage with her husband, and categorically denied that the money had come from Mr Berlusconi.
The ministerial code says gifts to ministers or their families should be notified to the permanent secretary - senior civil servant - in their department.
Labour MP Peter Kilfoyle, himself a former minister, said: "I think she's reached a point where she's got to consider resignation, frankly.
"The damage that's being done to herself and to the government is quite incredible, and I think she ought to reconsider her position."
Chairman of the Committee on Standards in Public Life, Sir Alistair Graham said the Cabinet Secretary was not the right person to investigate Ms Jowell's conduct.
He said the situation showed that the current regulations about investigating the conduct of ministers were "pretty bankrupt".