A new witness is expected this week to undermine thoroughly the case against the only person to be convicted of the Lockerbie bombing. New testimony will call into question evidence linking the Libyan Abdel Basset Ali al-Megrahi to the bomb that blew up Pan Am Flight 103 in 1988, his lawyers claim.

Megrahi, who has terminal cancer, is serving 27 years in Greenock prison for the bombing.

Appeal hearings are due to begin on Tuesday, and Megrahi's lawyers insisted this weekend they will go ahead as planned, despite speculation that he may be returned to Libya under the terms of a controversial prisoner transfer agreement, due to be ratified tomorrow.