The letter exposed the growing role of Saad bin Laden, the son of Osama, the al-Qaeda leader, as an intermediary between the organisation and Iran. Saad bin Laden, 29, has been living, apparently under house arrest, in Iran since the fall of the Taliban in Afghanistan in 2001.

The letter, which was signed by Ayman al-Zawahiri, al-Qaeda's second in command, was written after the US embassy in Yemen was attacked by simultaneous suicide car bombs in September.

Western security officials said the missive thanked the leadership of the Revolutionary Guards for providing assistance to al-Qaeda to set up its clandestine network in Yemen, which has suffered 10 al-Qaeda-related attacks in the past year, including two bomb attacks against the US embassy.

In the letter, al-Qaeda's leadership pays tribute to Iran's generosity, saying that without its "monetary and infrastructure assistance" it would have not been possible for the group to carry out its attacks.

The letter also thanked Iran for having the "vision" to help al-Qaeda establish new bases in Yemen after the organisation was forced to abandon much of its infrastructure in Iraq and Saudi Arabia.

There has been intense speculation about the level of Iranian support for al-Qaeda since the 9/11 Commission's report on the attacks against the US in 2001 concluded that Iran had provided safe passage for many of the hijackers travelling between Afghanistan and Saudi Arabia.

Scores of senior al-Qaeda operatives - including Saad bin Laden - sought sanctuary in Iran following the overthrow of the Taliban, and have remained in Tehran ever since.

Iran has been an important transit route for al-Qaeda loyalists moving between battlefields in the Middle East and Asia.

Western security officials have concluded that Iran's Revolutionary Guards have supported al-Qaeda cells, despite religious divisions between Iran's Shiite Muslim revolutionaries and the Sunni Muslim fighters from al-Qaeda.

Iran is also active in Yemen, which is Osama bin Laden's ancestral homeland.

■ Concerns over Iran's nuclear program were at the top of the agenda for a meeting in Washington yesterday between the Israeli Prime Minister, Ehud Olmert, and the US President, George Bush.

During his three-day stay, Mr Olmert hopes to get commitments from the US on Iran's nuclear program, after the president-elect, Barack Obama, said during the election campaign that he might be open to talks with Tehran.

Britain yesterday assured Iran that it was "100 per cent committed to diplomacy" to end the impasse over Tehran's nuclear ambitions, saying it expects Mr Obama's administration to follow the same approach.

Britain's Foreign Secretary, David Miliband, said in a speech that the sanctions supported by Britain were "not an attempt at regime change. And nor are they a precursor to military action."

Telegraph, London; Agence France-

Presse; Guardian News & Media