A Libyan envoy representing Col Muammar Gaddafi has been in London in the past few days for talks with British officials, the BBC has learnt.
Mohammed Ismail, an aide to one of Col Gaddafi's sons, was apparently told the Libyan leader had to give up power.
The Foreign Office said "in any contact that we do have we make it clear that Gaddafi has to go".
Meanwhile, the US says air attacks have destroyed about a quarter of the Libyan government's military's capabilities.
Following the defection on Wednesday of the Libyan Foreign Minister, Moussa Koussa, there were unconfirmed reports that other senior Libyans were defecting.
Al-Jazeera said the intelligence minister, deputy foreign minister and General People's Congress speaker were awaiting flights in Tunisia.
Oil Minister Shukri Ghanim has denied any intention to leave Libya. Ali Abdul Salaam Treki, who was recently named as Libya's permanent representative to the United Nations, is not accepting the post.
A government spokesman told reporters that they were all in Libya, but that it was also possible that some had left the country on a mission.