Justice Secretary Jack Straw ordered an immediate inquiry on Sunday into allegations that Scotland Yard anti-terrorist police secretly bugged an MP during private conversations with a constituent in prison.
The Sunday Times reported that Sadiq Khan, a government whip and Labour MP for Tooting, was bugged twice while meeting the constituent, who was in prison awaiting deportation to the United States to face terrorism charges.
The electronic listening device was hidden in a table at Woodhill prison in Milton Keynes and picked up conversations between the two in 2005 and 2006 about the latest developments in the US extradition request, the newspaper said.
Police have been forbidden to eavesdrop on politicians since a bugging scandal involving Labour Prime Minister Harold Wilson's government.
"I have ordered an immediate inquiry into this matter," Straw said in a statement. "Though I have no knowledge of the facts in this case, it is completely unacceptable for an interview conducted by an MP on a constituency matter - or on any other issue - to be recorded."
Muslim groups in Britain reacted with anger to the reports. "Today's revelations are simply appalling and raise a whole range of vital issues to do with confidentiality and how to hold to account the improper behaviour of senior police officers," Dr Muhammad Abdul Bari, secretary-general of the Muslim Council of Britain, said in a statement.