Gordon Brown gave his backing on Tuesday to Baroness Scotland, the attorney-general, after she was fined £5,000 for hiring an illegal immigrant as a housekeeper.

The Conservatives questioned the prime minister’s judgment, saying Lady Scotland was a Home Office minister three years ago when the government legislated to make employers responsible for checking the status of their staff.

Chris Grayling, shadow home secretary, said: “After this, we can’t see how Lady Scotland can credibly stay in her job.”

The peer later likened the matter to a minor motoring offence: “It’s like driving into the City and not paying the congestion charge ... not a criminal offence,” she told Sky News.

Mr Brown said he had consulted the cabinet secretary and decided to draw a line under the affair: “I have concluded that no further action is necessary given the investigation and action that has already been taken by the appropriate authorities and her unreserved apology.”

The attorney-general, who attends cabinet but is not a cabinet minister, had hired Loloahi Tapui, 27, as her housekeeper for six months. It emerged last week that Ms Tapui, a Tongan, had overstayed her student visa by five years.

Lady Scotland’s career was saved after the UK Border Agency accepted she had checked Ms Tapui’s documents when she hired her. The body imposed the fine because of her failure to keep any of the relevant documentation. The maximum penalty is £10,000.

Lady Scotland issued an apology: “I accept it is my duty to pay the fine and I have done so.”

Westminster Blog

It could be worse for the attorney-general. Not only has the UK Border Agency fined her only £5,000 (against an expected £10,000) but it has accepted her claim that she did check the documents of her (illegal immigrant) employee. Her mistake was the failure to keep those papers.

Her position had been under scrutiny since the story broke. On Sunday, Graham Stringer, a Labour MP, called for her resignation or sacking.

“The whole government is tarnished. People feel the government is passing laws it is not applying to itself,” said Mr Stringer.

The UK Border Agency said Lady Scotland had co-operated fully and it was satisfied that she had not employed an illegal worker knowingly.

“The UKBA is also satisfied that the employer took steps to check documents provided to her as proof of right to work in the UK,” it said.

“However, the law requires that employers must keep copies of documents proving the right to work in the UK and in this instance the employer failed to meet this requirement.”

The agency said the case should “remind all employers” that they were responsible for checking the immigration status of their staff and retaining proof of the relevant documents.