Mr Blair's office said the pair spoke by telephone on Saturday night, when the South African president was congratulated for his work in brokering the deal.

Downing Street added that Mr Mandela had confirmed that Libya's Colonel Muammar Gaddafi had agreed to hand over the suspects by 6 April.

"The prime minister welcomed it and said he was very grateful for his work towards this," said a spokeswoman.

United Nations Secretary-General Kofi Annan would now liaise with the Libyan government on the transfer details, she added.

The April date had earlier been confirmed in a letter delivered to Mr Annan by Libya's UN Ambassador, Abuzed Dorda.

Shortly after receiving the letter, the UN head said: "It looks as if we're there".

The US and UK Governments first demanded back in 1991 that the two Libyan intellligence agents face trial over the bombing of Pan Am flight 103, in which 270 people died.

Nelson Mandela announces Libya's intention to hand over the suspects

Mr Mandela had made public the news, by reading out the letter to the Libyan People's Congress.

Colonel Gaddafi told the meeting he had received enough assurances from Saudi Arabian and South African mediators to allow the handover of the two men for trial in the Netherlands.

Abdel Baset al-Megrahi

He said he would set no further conditions for the handover of Abdel Baset al-Megrahi and Lamen Khalifa Fhimah.

UK Foreign Secretary Robin Cook said he was encouraged by the news but warned : "I am not going to start taking anything for granted just yet."

British relatives of the Lockerbie bombing victims have praised the decision, although spokesman Dr Jim Swire called for the exact date of the handover to be kept secret to ensure the suspects' safety.

"These two are entitled to the presumption of innocence like anyone else," he said.

Sanctions must be lifted

The Libyan letter to the UN insists that the sanctions imposed in 1992 to try to secure the handover, be lifted within 90 days of confirmation that Libya has met with UN Security Council requirements.

Lamen Khalifa Fhimah

The US and Britain had given Libya until 26 March to surrender the two agents wanted for the bombing.

The two Western allies said if Libya failed to meet the deadline, they would seek tougher UN sanctions against the country.

Mr Mandela said the deal on the handover had been worked out with the help of Saudi Arabia's King Fahd and Crown Prince Abdullah.

Both South Africa and Saudi Arabia have been playing a leading role in diplomatic efforts to resolve the deadlock over the proposed trial of the two Libyan intelligence agents.