The Foreign Secretary, Robin Cook, and the US Secretary of State, Madeleine Albright, are expected to announce on Thursday that Britain and the United States are prepared to allow the two Libyans accused of the 1988 bombing of Pan Am 103 to be tried in the Hague. 

The former US president acted as an intermediary as Washington and London prepared to change policy over a trial venue. 

There has been an international stalemate since 1991 when the Libyans were charged with the explosion, which killed 270 people. The Libyans have refused to surrender for trial in either Scotland or the US because their government said they would not get a fair hearing. 

Washington and London have until recently insisted that they must be tried in Scotland or the US. 

The Scotsman can reveal that Mr Carter used his influence on behalf of African and Arab countries to contact the White House and State Department to see if a compromise could be reached. 

Lord Steel of Ettrick, the former Liberal leader, played a similar role as intermediary to sound out the British Government. 

In the past, the White House has informally used Mr Carter as a sounding board when it has explored ways of resolving tricky international dilemmas, and he helped broker a ceasefire in Bosnia. 

The most significant moves to break the logjam began after Labour was elected to power last May. 

In August last year, Lord Steel, the former Conservative MP Cyril Townsend, and Dr Jim Swire, whose daughter was killed in the bombing, met Libyan ministers in Tripoli. They received an assurance that the suspects would hand themselves over to the custody of the Arab League prior to a trial. 

The agreement fell through at a subsequent meeting of the Arab League when the Libyan leader, Colonel Muammar al-Gaddafi, sent junior officials on his behalf. 

In the meantime, the South African president, Nelson Mandela, acting on behalf of the Organisation of African Unity and the Arab League, had been pushing for a compromise. 

He made a high-profile call for a change of policy at the Commonwealth Heads of Government Conference in Edinburgh last October. 

In the same month, the Ugandan president, Yoweri Museveni, appointed one of his MPs to act as an emissary to seek a breakthough in Washington and London. 

The MP, John Kazzorra, asked Lord Steel and Mr Carter if they would work together to seek a solution. In the mid-1990s Lord Steel was president of the Liberal International, a global convention of liberal left parties to which the American Democrats are affiliated. It was through the Liberal International that Lord Steel got to know Mr Carter and Mrs Albright. 

By December, Mr Carter had approached the White House and US State Department and informed Mr Kazzorra that the obstacle to a third-country trial lay in London, not Washington. This was surprising news, because the US has always been a vehement opponent of allowing Libya to alter the trial venue. 

While pressure has mounted from African and Arab countries for a lifting of the United Nations sanctions imposed on Libya for its refusal to hand over the suspects for trial, the US has staunchly insisted they must be maintained. 

This July, Mr Cook held a secret meeting in London with the general secretary of the Arab League, Esmat Abdel-Meguid, as details of the compromise were being ironed out. 

Although Lord Steel never actually spoke to Mr Carter during the diplomatic negotiations, both men played a vital role in bringing about the policy U-turn that may result in a trial in the Hague. 

Lord Steel said yesterday: "I have always considered that this was a major international crime and that we would never get to the truth unless we got a trial. That has been my interest all along." 

There may still be significant obstacles to a trial taking place. The Libyans have agreed to stand trial in a neutral country under Scottish rules, but only if the jury is replaced by an international panel of judges. 

So far, contradictory messages have emerged from the British Government, suggesting either a single Scottish judge would preside, or that the jury would be replaced by a panel of Scottish judges. But neither of these options may prove acceptable to the Libyans.