Alan Decabral was shot twice in the head

But before he made it to the witness stand, Mr Decabral had been ordered by "gangsters" to "shut up or we will shut you up".

Immediately after the court case, in which Noye was found guilty and sentenced to life imprisonment, Mr Decabral went into hiding.

His marriage broke up because of the stress of the ordeal and he continued to suffer intimidation. He became "terrified" after three bullets were pushed through his letterbox with another warning.

On Thursday Mr Decabral was murdered in a car park in Ashford, Kent. Police say they are planning to interview Noye about the cold-blooded killing, although they have stressed the case is complex.

The murder raises concerns about the safety of witnesses in major criminal trials.

The Witness Protection Scheme is designed to look after key individuals who can provide essential evidence, generally in relation to the most serious offences.

Kenneth Noye will be questioned over Mr Decabral's murder

There is also the proviso that in testifying, the witness must face a substantial threat to their safety.

Most people who live under the scheme are "supergrasses" - criminals who turn on their underworld associates.

But victims of crime and innocent bystanders are also eligible. Danielle Cable, the fianc�e of Noye's victim Stephen Cameron, has been living under the scheme since she helped police identify Noye in 1998.

Only some police forces run dedicated witness support schemes - the Metropolitan Police, Greater Manchester, Northumbria, West Yorkshire, Hampshire, Strathclyde, Merseyside and the RUC in Northern Ireland.

Run on informal basis

Forces that don't have the same scale of criminal activity tend to rely on trained officers to provide witness support on an ad hoc basis.

Danielle Cable calls her mother from untraceable locations

Schemes are governed on an informal basis, unlike in countries such as the United States and Canada, which have legislated for formal protection programmes.

Because of the cost and complexity of the scheme, as well as the upheaval to witnesses, police only resort to it in extreme cases.

Individuals living under the scheme essentially trade in their old life for a new one. That means moving house - often to another part of the country - and changing one's identity. They also receive a lump sum of money.

Cut off from friends

Speaking earlier this year, Miss Cable said she has virtually severed communication with her friends and family.

"I have lost twice - Stephen and my old life," she told the Daily Mail. "I haven't seen two of my brothers since I was relocated, and I didn't see my mother for four months."

Simon Hughes MP had to persuad witnesses to testify in the Jamie Robe case

Phone calls are strictly limited and it takes six weeks for letters to reach her.

Miss Cable said police gave her just a few minutes to pack her bags and leave her old home. For the rest of her life she will live in fear of a contract killing, she said.

The programme was also deployed in the case of Jamie Robe, the 17-year-old who was murdered on a south London estate in 1997. Twenty people - witnesses and their families - were moved out of the area.

Yet witness protection does not have a 100% record. Last year, IRA supergrass Martin McGartland survived an attempt on his life after being shot six times in the stomach.

'Badly managed'

Mr McGartland had moved from Northern Ireland in the early 1990s and was living under a new identity in Whitley Bay, Tyneside.

Police find it hard to re-locate witnesses because of a lack of council housing

On other occasions, police have been criticised for poor management of the programme.

Thomas McCartney, whose testimony helped secure the convictions of three Ulster Volunteer Force men in Northern Ireland, accused the RUC of breaking their promise to look after him.

Mr McCartney, who is living somewhere in England, said the RUC failed to protect the homes he left behind and never gave him the new identity and passport he had been promised.

He said the �25,000 he was offered as compensation was "grossly insulting".