According to the April 2005 edition of Parliament's Register of Members' Interests, the Work and Pensions Secretary earned between £15,000-£20,000 as an adviser to the Organisation for Research and Technology, which he described as an international charity.
The Ministerial Code states that former ministers should inform the Advisory Committee on Business Appointments (ACBA) about business offers within two years of leaving office, to avoid any conflict of interest.
Mr Blunkett quit as Home Secretary in December last year in the wake of the so-called 'nannygate' affair.
Shadow Leader of the Commons Chris Grayling said this afternoon that he has again written to Tony Blair to challenge him to act over Mr Blunkett's conduct.
Mr Grayling said: "I am astonished that Mr Blunkett has broken the Ministerial Code on yet another occasion. This is getting beyond a joke.
"Tony Blair cannot claim the Ministerial Code is important and fail to act over these breaches."
In his letter, Mr Grayling said: "According to Mr Blunkett's entry in the Register of Members' Interests, he took up paid employment with a charitable organisation called the Organisation for Research and Technology. The appointment took place between the publication of the registers in January and April.
"He asked Lord Mayhew (chairman of ACBA) in their exchange of letters whether the Committee has a remit over charitable organisations, and was told that in the case of paid employment, it did.
"Nonetheless, I have spoken to the Committee this morning and have been told that Mr Blunkett made no attempt either to brief them about or to consult with them over this appointment.
"This appears to be a third clear example of a breach of the Ministerial Code by Mr Blunkett.
"Can you confirm to me please what action you intend to take over these breaches."
Earlier, Mr Blair had made it clear that he did not believe that Mr Blunkett should be disciplined for breaking the Ministerial Code because the breach did not affect his ability to do his job.
Mr Blair's views, relayed by his official spokesman, came after the publication last night of letters which showed Mr Blunkett was warned three times that he should consult with ACBA before taking up business appointments following his departure from Government.
Two of the letters touched on a job Mr Blunkett held with Indepen Consulting, a firm of business consultants.
This morning, Westminster watchdog Sir Alistair Graham, the chairman of the Committee on Standards in Public Life, said the letters left no doubt that Mr Blunkett had broken the ministers' code when he later joined the board of paternity testing firm DNA Bioscience in April.
But Sir Alistair made clear that the rules say it is up to the Prime Minister to decide how the breach should be dealt with.
Mr Blair's official spokesman told reporters at a regular briefing in London: "David Blunkett has accepted he made a mistake.
"The question is, does that mistake stop him doing his job? The Prime Minister's judgment is that it does not."Reuse content