UK Work and Pensions Secretary David Blunkett is facing calls to quit over claims he broke ministerial rules.
Mr Blunkett may have breached the ministerial code of conduct when he took a job with a DNA testing firm.
Mr Blunkett briefly became a director of DNA Bioscience for a two-week period before the general election but resigned after he was re-appointed to the Labour cabinet by Tony Blair.
But the official code suggests he should have sought the advice of the independent advisory committee on business appointments before taking up the job.
Mr Blunkett conceded he should have contacted the committee. But he said when he sought its advice on another occasion he was told it was voluntary. He has now asked the cabinet secretary to clarify the procedure.
But shadow leader of the House, Chris Grayling, said Mr Blunkett should resign. "There now needs to be an urgent inquiry into Mr Blunkett's conduct in relation to his business interests," he said.
"There are just too many questions gone unanswered for it not to be looked at properly," he told the Independent on Sunday.
"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 Grayling has also written to the Cabinet Secretary, Sir Gus O'Donnell, and to the Prime Minister demanding they investigate the breach. Mr Blunkett defended his actions.
A spokesman said: "As we have already stated, Mr Blunkett was involved with DNA Bioscience for two weeks, which fell during the General Election and while Parliament was prorogued. "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 has faced repeated controversy over his connections with the firm. Mr Grayling last week called for an independent inquiry into his relationship with the company after it was disclosed that he had bought shares in it in trust for his sons.
Mr Blunkett said there was no conflict of interest between the shareholding and his official duties as Work and Pensions Secretary.