The UN's European consultant on explosives, John Wyatt, has revealed that the fragment of circuit board crucial to the Crown's discredited case against Abdelbaset Ali Mohmed Al Megrahi could not have survived the explosion that was claimed to have brought down Pan Am 103.

Wyatt conducted 20 separate test explosions on identically mocked up suitcases, and has told the BBC that it was "unbelievable" such a fragment of evidence could have survived.

The findings echo those backed by UN Observer Hans Kochler, Professor Robert Black and the Lockerbie Justice Group, who challenged the Lord Advocate to demonstrate that the "scientifically implausible" theory stood up to scrutiny.

She declined to engage with the challenge.

"I do find it quite it extraordinary and I think highly improbable and most unlikely that you would find a fragment like that - it is unbelievable," said Wyatt.

"We carried out 20 tests, we didn't carry out 100 or 1,000, but in those 20 tests we found absolutely nothing at all - so I found it highly improbable that you would find anything like that, particularly at 10,000 feet when bits are dropping into long wet grass over hundreds of miles."

Wyatt has recreated the suitcase "bomb" which the Crown claimed destroyed Pan Am 103, using the type of radio in which the explosive and the timer circuit board were placed, according to the Crown theory. In each test the timer and its circuit board were obliterated.

The report - to be broadcast in full on Newsnight this evening- adds that the fragment of "evidence" was never subjected to chemical analysis or swabbing to establish whether it had in fact been involved in any explosion.