Downing Street has defended David Blunkett in a row over his directorship of a DNA testing firm before his re-appointment to the Cabinet.
Mr Blunkett held the post when he was on the backbenches
A spokeswoman said the work and pensions secretary has the full support of the prime minister.
The Tories are calling for an inquiry into a possible breach of rules.
Ex-ministers are meant to consult an independent advisory committee if they take up paid work within two years of leaving office.
Mr Blunkett said that "with hindsight it might have been better" to have consulted the committee before taking the job when he was between cabinet posts.
But the work and pensions secretary pointed out the consultation is voluntary and that he had believed he was acting within the code.
Shadow leader of the house Chris Grayling has led the call for an inquiry and for Mr Blunkett's resignation.
And the Liberal Democrat work and pensions spokesperson, David Laws, told the BBC he thought Mr Blunkett had, "done something wrong" and it was, "a serious error of judgement".
Mr Grayling has written to Cabinet Secretary Sir Gus O'Donnell and intends to write to Tony Blair on Monday about the matter.
"There now needs to be an urgent inquiry into Mr Blunkett's conduct in relation to his business interests," he told the Independent on Sunday.
"There are just too many questions gone unanswered for it not to be looked at properly.
"Mr Blunkett's position is now increasingly untenable. I really don't see how he can carry on as a Cabinet minister given all that's gone on in recent days."
Mr Blunkett took up the post at DNA Bioscience for a two-week period before the general election but resigned when he was re-appointed to the Cabinet.
He said that when he had previously consulted the advisory committee on an unrelated matter in March, he had had a response pointing out that "the voluntary character of the scheme for former Ministers is exactly that: it is voluntary".
With regard to the non-executive role at DNA Bioscience, his statement says: "Had Mr Blunkett not returned to government, there would have been time to seek clarification in regards to the committee, accepting as he does its helpful role.
"Mr Blunkett believed that he was acting within the Ministerial code, but with hindsight it might have been better if he had written to the committee prior to parliament returning."
Mr Blunkett says he has asked the secretary to the Cabinet to clarify the procedure "in order to avoid any confusion in the future".
Meanwhile some Sunday newspapers reported that a row has broken out between Mr Blair and Mr Blunkett, after Number Ten argued that the plans for welfare reform did not go far enough.
The Observer and the Mail on Sunday said Mr Blunkett had sent the prime minister a letter allegedly complaining over Mr Blair's interference in the affairs of his department.