Attorney General Baroness Scotland was under growing pressure today after a ministerial aide quit over her breach of immigration law.
Sources confirmed that Labour MP Stephen Hesford, parliamentary aide to the Government law officers, had stepped down this morning.
In a letter to Gordon Brown, he apparently said that although he had "great personal regard" for Lady Scotland, he felt she should have left her post as a matter of principle.
Mr Hesford, a parliamentary private secretary and the MP for Wirral West, wrote: "My decision comes about because, as an aide to the Law Officers, whilst I have great personal regard for the Attorney General, I cannot support the decision which allows her to remain in office.
"In my view, the facts of the case do not matter. It is the principle which counts, particularly at a time when the public's trust of Whitehall is uncertain to say the least.
"We have to be seen to be accountable."
Lady Scotland was fined £5,000 yesterday after it emerged that she had employed a Tongan housekeeper who had no right to work in the UK.
The UK Border Agency found that the peer had failed to keep copies of documentation which seemed to suggest 27-year-old Loloahi Tapui was legal.
The Prime Minister refused to sack Lady Scotland, but she risked inflaming the controversy further by comparing her "inadvertent" mistake to forgetting to pay the Congestion Charge in London.
Mr Hesford told BBC Radio 4's The World At One: "The Attorney General is the senior law officer, she is the chief adviser on legal matters to the Government and as a matter of principle I just think it is wrong that someone who has the honour of serving in that office should remain, considering the circumstances of what happened yesterday."
He said Lady Scotland's comments that her offence was similar to not paying the congestion charge highlighted why she had not taken the right decision in considering her position.
Mr Hesford said: "Almost regardless of the facts of the case - as I say I am sorry that these things have happened - but if you have the honour to serve you also have the principles to uphold that if things go wrong, you don't stay in a position that could possibly make your job harder, which this undoubtedly will, or embarrass the Government which you serve."
Mr Hesford insisted his resignation was no reflection on the performance of Gordon Brown, who he said was doing a "fantastic job".
And he added: "I see this as a personal honour situation, which otherwise a very good minister finds herself in. I think the law officer's position at the top of the legal tree is a peculiarly sensitive position and requires extra sensitivity."
Responding to news of Mr Hesford's resignation, the Prime Minister said he believed Lady Scotland had acted "in good faith" and should not lose her job over the civil fine she was given.
Mr Brown told BBC Radio 5 Live: "If Baroness Scotland had wilfully or knowingly employed any illegal worker, then obviously she wouldn't be in her post.
"If Baroness Scotland had not gone to the trouble of checking the documents that were produced by her employee to show she was legally allowed to remain in this country, then obviously she could not have stayed in her post."
But he added: "I have got to accept that Baroness Scotland acted in good faith... I take this very seriously indeed, but when you look at the facts of the case, she has been misled by an employee who has given her wrong information.
"She has not knowingly employed somebody in any unlawful way."
A member of the public, identified only as Mark, who called the programme, told Mr Brown he was misjudging voters' mood on the issue, adding: "If she can't follow the law which she helped put through Parliament, she can't hold the post."
But Mr Brown responded: "I have got to look very carefully at whether we are being fair to people or not being fair to people.
"She has paid the fine and apologised unreservedly. I have told her of my feelings about what has happened, but I have got to ask if someone should lose their job for failure to keep a copy of documentation.
"In all other respects, she did everything she would be expected to do."