An investigation by BBC's Newsnight has cast doubts on the key piece of evidence which convicted the Lockerbie bomber, Abdelbaset Ali al-Megrahi.

Tests aimed at reproducing the blast appear to undermine the case's central forensic link, based on a tiny fragment identified as part of a bomb timer.

The tests suggest the fragment, which linked the attack to Megrahi, would not have survived the mid-air explosion.

Newsnight has been reviewing that evidence, and has exposed serious doubts about the forensics used to identify the fragment as being part of a trigger circuit board.

The fragment was found three weeks after the attack. For months it remained unnoticed and unremarked, but eventually it was to shape the entire investigation.