A forensic scientist has denied that Dyfed-Powys police had accidentally mixed up exhibits allegedly linking farm labourer John William Cooper to a double murder.

But Dr Philip Avenell said the way evidence was handled would not happen today.

Dr Avenell, of LGC Forensics, the country’s largest independent forensic examination firm, led a team that examined items police believed were connected with the murders of holidaymakers Peter and Gwenda Dixon, who were shot dead as they walked the Pembrokeshire coast path in 1989.

Dr Avenell said in April 2009 his team discovered what was thought to be a bloodstain in the hem of a pair of green shorts found in the bedroom of Cooper’s then home in January, 1998.

Tests showed it contained the DNA of two women.

He sent the test results to Professor David Balding, a specialist in statistical genetics at University College, London. He estimated there was only a 999million to one chance that it did not contain the DNA of Julie Dixon, the murdered couple’s daughter.

Dr Avenell said Cooper’s DNA was also found on the shorts, which the prosecution say were stolen from the murder scene near Little Haven.

Tests on a shotgun found in a hedgerow near Cooper’s then home in St Mary’s Park, Jordanston, revealed the DNA of Peter Dixon by a one in one billion chance against.

Dr Avenell told Swansea crown court the firm took rigorous measures to ensure that exhibits did not come into contact with each other to avoid cross contamination.

Mark Evans, representing Cooper, claimed that had not been the case between 1989 and 2007, when the shorts were still in west Wales.

Dr Avenell said he and Det Inspector Glyn Johnson had reviewed the entire records of the exhibits, how they were stored and who handled them, and he was “completely satisfied” that blood had not been transferred from one to another.

Mr Evans said Cooper had been arrested in January 1998 and later jailed for 16 years for 30 burglaries and a robbery.

In preparation for the trial, police had used the old Gulf refinery to display thousands of items they thought were connected with crime scenes.

Mr Evans said it was now known that a heavily bloodstained rope used to tie the hands of Peter Dixon before he was shot had ended up in Box 36 along with items connected with the burglaries.

And the green shorts (pictured) had been stored in Box 40, which also contained material linked to the crimes Cooper was jailed for later that year.

“None of the 1989 exhibits should have been mixed with 1998 exhibits taken to the Gulf refinery,” said Mr Evans.

Mr Evans said the same items were later moved to Cawdor Barracks and again put on display. The scene was video recorded and it showed several items had been taken out of their evidence bags and placed on tables.

A balaclava and a jacket, allegedly used by Cooper during a robbery and later discarded in fields near Sardis, were actually touching each other.

“There is no control over contamination whatsoever,” said Mr Evans.

“There does not appear to be,” replied Dr Avenell, who said that under today’s standards the exercise would not have happened.

Mr Evans said the green shorts were in a box in the same room at Cawdor. Dr Avenell said he understood that to be true but that they had not been taken out of the bag they were in.

Mr Evans said in August 2006 a detective had noticed the bag containing the shorts was no longer sealed and no-one seemed to know how long it had been in that state.

“It could have been open for eight years,” said the barrister.

Mr Evans said the Dixons’ clothing had been sent at first to the Home Office’s forensic science laboratories in Chepstow. In October 2008, a forensic scientist found a plastic bin containing blood stained clothing from both Peter and Gwenda Dixon, mixed together and not in bags and along with “degrading material.”

“How could that happen?. Something has gone very seriously wrong,” he said.

Dr Avenell said he could not comment on how the laboratory operated but LGC would not store items in that way.