A senior aide to Colonel Gaddafi's son, Saif al-Islam has been in London in the past few days for talks with British officials, the BBC understands.

The Foreign Office said that in all its contacts with Libyan officials, it had made it clear that "Gaddafi has to go".

News of the secret contact came after the defection to Britain of Libya's former foreign minister, Moussa Koussa.

Meanwhile, forces loyal to Colonel Gaddafi have succeeded in halting an advance by the rebels.

James Robbins reports.