LONDON (AFP) - Britain's immigration minister resigned on Saturday after it emerged that he had employed a cleaner who was in the country illegally, Prime Minister David Cameron's office said.

Downing Street said there was no suggestion Mr Mark Harper, a junior minister in the Home Office, knew the cleaner was an illegal immigrant, adding that the woman had given him false papers.

Mr Harper had been overseeing the passage of new immigration laws, a key issue for Cameron's Conservative party ahead of elections in May 2015.

"Mark Harper offered his resignation after he was informed that his cleaner did not have indefinite leave to remain in the United Kingdom, despite having shown him documents claiming she did," a Downing Street spokesman said in a statement.

"He immediately notified the prime minister, who accepted his resignation with regret. There is no suggestion that Mr Harper knowingly employed an illegal immigrant." It was not immediately clear which country the cleaner was from.

Mr Harper said in his resignation letter that he had employed the cleaner in 2007 after making background checks but did not carry out further checks until last month, despite being appointed minister in 2012.

He then found that the cleaner did not have leave to stay in the country.

"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," Mr Harper wrote.

"I am sorry for any embarrassment caused."

Cameron said he was "very sorry indeed to see you leave the government" but added that Mr Harper had taken an "honourable decision." Cameron's centre-right Conservatives face a growing electoral threat from the anti-immigration, anti-EU UK Independence Party.

The issue has heated up sharply since Bulgarians and Romanians were given full rights on Jan 1 to free movement in the European Union, stoking claims in Britain and elsewhere they would take local jobs or sponge off the welfare system.

The government's efforts to push a new immigration bill through parliament have had a bumpy ride, with Conservative rebels saying it does not go far enough despite the inclusion of measures such as stripping naturalised terror suspects of British citizenship.