The Secretary of State for Work and Pensions was accused of breaking rules for ex-ministers when he failed to notify an independent watchdog that he had taken a directorship with DNA Bioscience.
He conceded that "with hindsight, it might have been better" to have sought advice on whether to accept the post."
Seizing on his admission, the Tories called for him to step down and demanded an investigation by Sir Gus O'Donnell, the Cabinet Secretary, into whether be broke the ministerial code.
The controversy left Mr Blunkett facing a new fight for his Cabinet future, 10 months after the storm over the fast-tracking of a visa for his former lover's nanny forced his resignation as Home Secretary.
Chris Grayling, the shadow Leader of the Commons, wrote to Sir Gus yesterday calling for the events to be investigated both by him and by an independent inquiry.
"There are so many question marks in relation to Mr Blunkett's judgement over this, I believe that his position has become untenable," he said.
"Unless something pretty dramatic happens to clear his name, then I don't think we can have confidence in him as a Cabinet Minister."
David Davis, the shadow Home Secretary, told Sky: "When he first resigned, when I was involved in that, I made a very clear point of not actually attacking or criticising him on things which were personal to him and things which were not covered by the ministerial code.
"If he has admitted to a breach of the ministerial code, I think he would have to step down again, but I haven't seen the details so I cannot comment authoritatively."
The controversy centres on Mr Blunkett's decision to accept a directorship of DNA Bioscience in April. He stepped down a fortnight later when he returned to the Cabinet, although he retains a 3 per cent stake in the company in trust for his three elder sons.
He appears not to have abided by rules which require former ministers to notify the independent Advisory Committee on Business Appointments about any job they wish to accept within two years of leaving office.
Mr Grayling said an investigation should cover whether the ministerial code was breached, whether it was right for Mr Blunkett to accept the job knowing he was likely to return to the Cabinet within weeks and whether it was proper for him to have a family stake in DNA Bioscience when it was pitching for Government contracts.
A spokesman for Mr Blunkett 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 [suspended].
"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."