A forensic scientist warned twice that exhibits in the Stephen Lawrence case could become contaminated in paper evidence bags, a court heard today.
Adrian Wain told police that the sacks and sticky tape used to seal them could degrade and therefore let in fibres or hairs.
A jury at the Old Bailey was told that Mr Wain wrote to officers in July 2001 to raise concerns after he had been asked to examine two items of Mr Lawrence's clothes for fibres.
Today he told the jury: "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."
Gary Dobson, 36, and David Norris, 35, deny taking part in the gang attack that killed Mr Lawrence in Eltham, south east London, in 1993.
The prosecution alleges that microscopic fibres, hair and blood found on clothes seized from their houses prove they took part in the murder.
Tiny traces were found by a cold case squad that began work in 2007.
Today Timothy Roberts QC, for Dobson, also read part of a police report written by Detective Chief Superintendent Barry Webb in 1999, which said: "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."
The jury was also told that Mr Wain decided against examining the suspects' clothes for fibres from Mr Lawrence's clothing.
He 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."
Mr Lawrence was wearing five layers of clothing on his top half when he died, and Mr Wain ruled out looking for fibres from the lower layers which could have been transferred.
He told Mr Roberts: "I don't believe I ever considered looking at undergarments for fibre transfer.
"I don't believe it was pertinent to current thinking at that time."
Mr Roberts told him that Mr Lawrence's clothes had been "jumbled together" when seized by police at the hospital after he died.
He said: "We can imagine a general mess of fibres and sticky body fluids all jumbled up together and so we can imagine that each of these five layers was likely therefore to have been exposed to contamination from the other garments."
Mr Wain replied: "Yes, I think that's inevitable."
The court has already been told that the cold case team examined a jacket belonging to Dobson with a microscope and found a tiny blood stain measuring 0.5mm by 0.25mm.
Mr Wain said that using a microscope to search an exhibit was "very rare" in 1993, and that when DNA testing was introduced the following year the minimum size of stain that they could have analysed was 3mm square.
Earlier his then-assistant Yvonne Turner told the jury that debris found in evidence bags would not have been examined at that time.
Two hairs with a DNA link to Stephen Lawrence - one 1mm and one 2mm long - were found by the cold case team in an evidence bag used to store Norris's jeans.
Ms Turner did not find any hair on them in 1993, but said it was "unlikely" that such tiny hairs would have been of interest anyway.
The trial was adjourned until tomorrow.