A piece of circuit board thought to have formed part of the bomb that blew up the Pan-Am aircraft over Lockerbie and which supported the thesis that linked Malta to the tragedy is unlikely to have survived the explosion, according to fresh tests by a British bomb expert.

The expert, a UN European consultant on explosives, John Wyatt, recreated the explosion 20 times, using a similar circuit board and timer and the parts were pulverised every time.

Talking to The Times yesterday, he said every test left absolutely no fragments like the one found at Lockerbie and which was used to implicate Libya and Malta in the whole affair.

The 1988 attack on Pan Am flight 103 over the Scottish town of Lockerbie killed 270 people.

"It was highly improbable to the point of making it unlikely" that such a fragment could have survived the blast, Dr Wyatt said.

The revelation undermines a key element in the prosecution's case against Libyan Abdelbaset Ali al-Megrahi, who was found guilty of the Lockerbie bombing. Last summer, Mr al-Megrahi was released from a Scottish jail on compassionate grounds after he was diagnosed with terminal cancer.

"We conducted 20 tests, 19 of which were indoors to make sure we could collect all the evidence. We even painted the circuit board bright yellow to make it easier to identify any fragments among the debris. In no circumstance did we find any fragment," Dr Wyatt explained.

The explosive tests were conducted in stages and in a controlled environment, which would have made it very easy to collect all the evidence.

"We tried exploding the device on its own; in a radio similar to the one it was supposed to have been planted in; in a suitcase with and without clothes; surrounded by other suitcases and, eventually, in a container. In all tests, the timer and the circuit board were completely destroyed," Dr Wyatt said.

The fragment found in Lockerbie had not been in ideal forensic conditions, he added, because the explosion happened at a height of 10,000 feet and the debris fell over a large area in the Scottish wilderness. "This increases the improbability of finding a fragment that was part of the bomb itself," he said.

Dr Wyatt, who has more than 25 years experience in the British army, mostly as a bomb disposal officer, conducted the tests for BBC's current affairs programme, Newsnight, which will be broadcast tomorrow.

When asked by The Times, the expert would not say whether his analysis would lead him to suggest the fragment could have been put there after the explosion. "That is not for me to say but it was very, very improbable for such a fragment to be found," he said.

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. The fragment was embedded in a charred piece of clothing, which was marked with a label saying it was made in Malta.

The Malta lead raised the question as to who would have bought the clothes.

The investigation zoomed in on the Sliema outlet Mary's House and shopkeeper Tony Gauci, who was the other key element in the prosecution's case, identified Mr al-Megrahi.

However, serious doubts were cast on Mr Gauci's testimony because the identification of Mr al-Megrahi only came years later after the witness had seen him pictured in a magazine as a Lockerbie suspect.

In fact, over the past years, the credibility of the main thesis that saw Mr al-Megrahi being convicted was seriously called into question and it was believed the appeal launched by the former Libyan secret service officer could prove this. However, to obtain his release, he had to relinquish the appeal.

This led several people, including Scottish relatives of people who died in the atrocity, to call for a fresh inquiry.

The call was never taken up, not even by the Maltese government, which, many believe, should lead the fight to clear Malta's name from the bombing implication.