Mr Blunkett's letter, marked "Private and confidential", went to the head of the Advisory Committee on Business Appointmentsdays before this newspaper reported that he had broken the rules on ministerial conduct.
The disclosures appear to contradict both Mr Blunkett's claim in his resignation statement that he was "guilty of a mistake" that he had wished he had "spotted earlier" and Tony Blair's assertion that his close political ally had not been guilty of any "wrongdoing".
Mr Blunkett was fully aware that he had breached the code. In his letter, Mr Blunkett set out his version of events surrounding his appointment to a DNA testing firm in an apparent attempt to dodge political fallout.
But in a reply the next day, Lord Mayhew, the chairman of the committee, warned the minister that it would tell the truth about his breach of the ministerial code "if inquiries were made". Mr Blunkett resigned as Secretary of State for Work and Pensions after The Independent on Sunday revealed last week that he had broken the code.
The revelation about Mr Blunkett's letter comes after Mr Blair told MPs that his cabinet colleague had left the Government "with no stain of impropriety against him whatever".
In a further setback, Mr Blair is criticised today by the watchdog responsible for maintaining standards on public life, on the grounds that his administration is seen as "weak" on tackling sleaze.
Sir Alistair Graham, the chairman of the Committee on Standards in Public Life, has accused the Prime Minister of failing to show leadership in enforcing ethics in public life, and warned that the "whole political class" now faces lack of public confidence over standards.
Saying he did not agree with Mr Blair that there had been no " impropriety" in the Blunkett affair, Sir Alistair accused the Prime Minister of giving a "rather obscure and muddled message on standards".
In an exclusive interview, he called for an immediate review of the Ministerial Code of Conduct and said ministers should no longer be able to ignore advice on taking business appointments.
Sir Alistair also questioned whether the Prime Minister, as the enforcer of the Ministerial Code, should have taken no action when it emerged that Mr Blunkett had broken the rules governing ministerial conduct.
He is discussing with his committee the launch of an official investigation into standards in public life.
"I think it would be very much better if the code of conduct did spell out ... that when people signed up to the code of conduct on taking office they were also spelling out what their behaviour will be when they leave office," he said.
He warned that the Government should give a firmer message on ethical standards, adding: "These issues in the end are always matters of leadership."
Mr Blunkett's aide last night defended his actions, but declined to say when the minister knew of the breach and whether he now regretted not immediately admitting it in public. Meanwhile, a rash of former ministers last week ensured their appointments were properly recorded.
* The Mail on Sunday has reported that Cherie Blair was guest of honour at a garden party thrown by Nighat Awan, whose brothers run DNA Bioscience, the firm at the heat of the Blunkett scandal. She dined at the party, in 2004, with Pervaiz Naviede , the man who ultimately controls DNA Bioscience. And last night Tessa Jowell faced questions over the timing of a declaration of details of her husband's business dealings in Iran.
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