The British Prime Minister Tony Blair has warned Iran that harbouring al Qaeda operatives would be "entirely unacceptable".
Britain has also discussed with the United States reports that a small cell of Osama bin Laden's militant group in Iran may have directed last week's suicide bombings in Saudi Arabia in which 34 people died, including eight Americans, Blair said on Thursday.
Iran denies US accusations that it shelters al Qaeda operatives.
"We have said very clearly to the Iranian government that harbouring al Qaeda would be entirely unacceptable," Blair told a monthly news conference at his Downing Street offices. Unlike the United States, its close ally, Britain has full diplomatic relations with the Islamic republic.
"I hope very much, if they are indeed harbouring al Qaeda operatives, that they yield them up because these people...are dangerous people," he said.
US Defence Secretary Donald Rumsfeld said this week there was "no question" that some al Qaeda members were in Iran, and that there was some speculation about their involvement in the bombings in Saudi Arabia.
Al Qaeda is blamed by Washington for the attacks on U.S. cities on September 11, 2001.