The former chief forensic scientist in the Stephen Lawrence murder investigation has told the Old Bailey he was concerned that crucial evidence had become contaminated.
Adrian Wain said that by the late 1990s sellotape used to seal bags of clothing had become ineffective.
Prosecutors say evidence on the clothes of Gary Dobson, 36, and David Norris, 35 links them to the 1993 killing.
Mr Dobson and Mr Norris deny murder and say police contaminated the evidence.
Mr Wain told the jury he had warned police in July 2001 that the sacks and sticky tape used to seal exhibits could degrade and therefore let in fibres or hairs.
He said: "I was aware that items had been in and out of the laboratory. I didn't have control of them outside the laboratory. I didn't know whether they'd been in the same location outside the laboratory.
"I knew that the packaging was deteriorating, I knew that the seals were deteriorating. I had concerns about contamination."
"Reluctant" to look
Timothy Roberts QC, who is defending Gary Dobson, also read the court part of a police report written by Detective Chief Superintendent Barry Webb in 1999.
It read: "The original Sellotape seals used when the items were seized in 1993 have become so inefficient that in Adrian Wain's view in the event of alien blood cells being found on the suspects' clothing in any subsequent examination he would be unable to rule out the possibility of contamination having occurred at the point of storage."
Earlier Mr Wain said he had been "reluctant" to look for fibres on clothes seized from two men suspected of murdering Stephen Lawrence.
He said he did not believe any fibres would be left because of the two-week gap between the stabbing and the moment the clothes were found.
Mr Wain said: "Given that there was this two-week gap between the offence and the seizure of the clothes I was reluctant to do the transfer of fibres from Stephen Lawrence on to the suspects.
"That and with the fact that I understood the attack was brief and contact was minimal."
The prosecution allege that microscopic fibres, hair and blood found on clothes seized from the houses of Mr Dobson and Mr Norris proves they took part in the murder.
The evidence was uncovered during a cold case review that began work in 2007.
However, the defendants say the evidence was transferred from Mr Lawrence's belongings onto the men's clothing because of police handling errors.
Mr Wain said in 1993 he first examined the suspects' clothes for blood, and that he restricted his research to fibres on Mr Lawrence's outer layers of clothing.
Tiny blood stain
The cold case team expanded their search to look for fibres from Mr Lawrence's red T-shirt which he was wearing under several layers of clothes, the court has already been told.
The team also examined Mr Dobson's jacket with a microscope and found a tiny blood stain measuring 0.5mm by 0.25mm.
Mr Wain said it was not normal to search clothes for clues with a microscope. At that time, before DNA testing became standard, only larger visible stains could be used as evidence.
He also said the "debris" from evidence bags was not routinely searched.
Bag debris was later found which has also produced evidence in this case.
Mr Lawrence, 18, a black A-level student, was forced to the ground by a group of white youths at a bus stop in Eltham and then stabbed twice, the prosecution says.
The trial continues.