Barnaby Mason Diplomatic correspondent

The second, and more crucial part, is a Libyan letter to the Security Council admitting responsibility for the attack.

The letter is a vital hurdle that Libya has to overcome before United Nations sanctions can be lifted.

At the moment, they are simply suspended and could in theory be re-imposed.

Tortuous negotiations

If all happens as planned, it will be a significant step towards closing the UN chapter of the lengthy Lockerbie saga, although it will not be quite the end of the story.

An alleged Libyan intelligence agent was convicted of the Lockerbie bomb attack in 2001.

Since then, there have been tortuous negotiations on whether and how the Libyan government itself will accept responsibility.

Those negotiations - between government officials of Libya, the United States and Britain - have moved in parallel with the talks between the Libyans and lawyers representing the families.

The wording of the Libyan letter to the Security Council is crucial.

A Libyan official was found guilty of the attack; our government is responsible for Libyan officials; therefore the government accepts responsibility for his act

It seems unlikely to be a clear admission that the Libyan state carried out the bombing.

One American lawyer said he expected an unsatisfactory statement in diplomatic legalese that could be interpreted in various ways.

The Libyans have been using the phrase "civil responsibility".

Their argument is: a Libyan official was found guilty of the attack; our government is responsible for Libyan officials; therefore the government accepts responsibility for his act.

Libya has been anxious to avoid an admission of criminal liability that might open the way to future prosecutions of senior officials, even of Colonel Gaddafi himself.

Nevertheless, it is assumed that the Libyan letter will go far enough for the United States and Britain to allow the passage of a Security Council resolution lifting UN sanctions.

Sahara plane bombing

It is possible that a last minute objection by France will delay the carefully choreographed process.

The French foreign ministry says that before sanctions are lifted it wants substantial progress towards equal compensation for the relatives of the victims of another bomb attack blamed on Libya.

That attack brought down a French airliner in the Sahara in 1989, killing 170 people.

It was linked with the Lockerbie bombing in Security Council resolutions imposing sanctions on Libya in 1992 and 1993.

Four years ago, a French court convicted six Libyans of the attack in their absence, including the brother-in-law of Colonel Gaddafi.

It is not clear how serious the French obstacle to a new Security Council resolution will turn out to be

Libya agreed to pay compensation, but the amount for each family was less than $200,000 - compared with a potential $10m each for the Lockerbie families.

It is not clear how serious the French obstacle to a new Security Council resolution will turn out to be.

But in any event, the United States will maintain for the time being its own more severe economic sanctions on Libya - some of which predate the Lockerbie bombing.

The Bush administration is still not convinced that Libya has stopped backing extremist groups and has accused it of trying to get hold of chemical weapons.

The payment of the second slice of compensation to the victims' families is tied to the lifting of US sanctions.

Gaddafi's rehabilitation

And the last $2m dollars will be handed over only when Washington removes Libya from its list of states sponsoring terrorism.

The Libyans hope the families will press Washington to take this action - but some are still bitter at the thought of rehabilitating Colonel Gaddafi into the community of civilised nations.

All the same, his gradual return to respectability continues.

He condemned the suicide hijackings of 11 September, 2001, and has co-operated in the hunt for those responsible.

In the new global atmosphere, Colonel Gaddafi seems to realise he must signal he is abandoning his past methods in order to play the larger international role he craves - especially in Africa.