British Foreign Secretary Mr Jack Straw said he would visit Iran to try and bring the Islamic republic into an international coalition against terrorism.
Mr Straw will next week make the first visit to Tehran by a British foreign minister since the Islamic Revolution in 1979. "It's important to build alliances with every country that we can," Mr Straw said.
He also denounced as totally unacceptable proposals by Afghan clerics that Osama bin Laden, prime suspect in last week's US attacks, leave Afghanistan voluntarily.
Mr Straw's planned trip follows what British Prime Minister Mr Tony Blair called a remarkable conversation yesterday with Iranian President Mr Mohammad Khatami.
Mr Straw had been planning to visit Iran later in the year, but the conversation between Mr Blair and Mr Khatami and a telephone call between Mr Straw and Iranian Foreign Minister Mr Kamal Kharrazi saw the trip being brought forward.
Mr Khatami condemned last week's suicide airliner attacks on New York and Washington and called for better relations between Britain and Iran.
But Iran has said it will not allow its airspace to be used for any attacks on neighbouring Afghanistan.
Tehran's moderate leadership is anxious to reach out to the West, but at the same time it can be expected to make clear to Mr Straw it is deeply concerned about a major US-led assault on Afghanistan.