Immigration minister Mark Harper has resigned from the government after it emerged his cleaner did not have permission to work in the UK.
Mr Harper notified Prime Minister David Cameron, who accepted his resignation “with regret”, Number 10 said.
It added there was “no suggestion” the 43-year-old Conservative MP for the Forest of Dean had “knowingly employed an illegal immigrant”.
Fellow Tory James Brokenshire has been appointed the new immigration minister.
In his letter to the PM, Mr Harper said he would continue in his role as MP.He also explained in the letter that he had first made checks on his cleaner in 2007 and again in 2012.
A year later, he said, he talked a lot about employers and landlords carrying out “reasonable checks” on workers.
Given this focus, he said, he thought it “prudent to check that all my documents were in order for my cleaner”.
“I undertook an extensive search to locate the copies of documents I had taken but unfortunately I was unable to locate them,” he wrote.
Last month, he asked his cleaner for further copies but when his private office checked the details with immigration officials, it was found she did not have indefinite leave to stay in the UK.
He was told this on Thursday and said in his letter that he immediately told Home Secretary Theresa May.The matter was now with immigration enforcement, he added.
“Although I complied with the law at all times, I consider that as immigration minister, who is taking legislation through Parliament which will toughen up our immigration laws, I should hold myself to a higher standard than expected of others,” he also wrote.
“I have always believed that politics is a team game, not an individual sport,” he went on.
“Under the circumstances, I have therefore decided that the right course is for me to return to the backbenches. I am sorry for any embarrassment caused.”
In his reply, Mr Cameron said Mr Harper had “taken an honourable decision” and he hoped to see him return to the frontbench “before too long”.
“I understand your view that, although you carried out checks on your cleaner, you feel that you should hold yourself to an especially high standard as immigration minister,” Mr Cameron wrote.
He added that Mr Harper’s energy and loyalty had always been appreciated, adding that it was “typical of you that you should be so mindful of the wider interests of the government and the party in reaching the decision that you have”.Home Secretary Theresa May said: “Mark has been an excellent minister and he can be proud of the role he has played in sharply reducing immigration to Britain.
“I am extremely sorry to lose him from the Home Office but I am confident that he will be able to bring his considerable talents to government as a minister again in future.”
BBC home affairs correspondent Danny Shaw tweeted: “Mark Harper will probably be best remembered for the “go home” ad vans & measures to clamp down on benefit claims from EU immigrants.”
The vans referred to by our correspondent were driven around parts of London carrying a message for illegal immigrants to “Go home or face arrest”.
The Home Office initiative resulted in few calls to the helpline number provided.
At the start of this year, when work restrictions for Bulgarians and Romanians were lifted, Mr Harper insisted the benefits system had been “tightened up” so people coming to the UK did so to work, not claim benefits.
Mr Harper was appointed immigration minster in September 2012. He has also held the post of minister for political and constitutional reform and shadow minister roles for defence and disabled people.
Before becoming an MP in 2005, Oxford University-educated Mr Harper established his own chartered accountancy business in the Forest of Dean, in Gloucestershire.
New immigration minister James Brokenshire is MP for Old Bexley and Sidcup and was formerly a Parliamentary under-secretary of state at the Home Office.