The Old Bailey trial, which lasted almost six weeks at a cost of nearly pounds 1m, was halted after the Recorder of London, Judge Lawrence Verney, ruled that the evidence of Renwick Dennison and Stuart Warne, the main prosecution witnesses, could not be relied on.
At an earlier trial, Warne, the main link in the alleged plot between the drug dealers and the Irishmen, and Renwick, allegedly one of the hired killers, had both admitted conspiracy to murder and drugs charges, and had been jailed for life.
Yesterday Timothy Langdale QC, for the prosecution, said that without their evidence he could not go on with the case.
The prosecution had outlined a plot involving the four men in the dock, Warne and Renwick, and two other men who had never been caught.
The victims of the alleged plot were John Dale, described as 'the object of dislike and hatred apparently because he was in the habit of ripping people off in drug deals', and David Norris, a police informant. Both were shot outside their homes in April 1991. Mr Dale, 43, survived, but Mr Norris, 46, died.
Two of the men cleared yesterday - Patrick Doherty, 35, and George McMahon, 46, both of south London - were allegedly involved in drugs and commissioning the contract killings. The other two were Terence McCrory, 30, of Belfast, and John Green, 32, of Falkirk.
All denied conspiring with others to murder Mr Dale. Mr McCrory, Mr Green and Mr Doherty denied a similar conspiracy to murder Mr Norris. Mr Doherty and Mr McMahon denied conspiracy to supply cannabis resin. The hitmen had been hired, the prosecution alleged, through Northern Ireland drug dealers, who were promised access to cheap cannabis in return for fulfilling contracts. The court heard that the two men, who have not been arrested and are on the run, had allegedly helped to organise the murder squad. They were named in court as Thomas McCreery and Stephen Pollock, a Kent-based Irish drug dealer.
Mr McCreery allegedly came to London from Belfast after he was shot for supplying Catholics with drugs. The court was told they had used the BR Red Star parcel service to Belfast to send cannabis, which they had bought from London gangsters.
Warne had told the jury that Mr Doherty said he was prepared to pay pounds 35,000 to have Mr Norris killed and claimed Mr Norris had 'grassed up a number of firms'.
He had described a meeting with Mr Doherty in a south London public house when Doherty used his mobile telephone to order pounds 20,000 in cash to pay for the Dale killing; it arrived 30 minutes later, accompanied by an armed guard, he said.
The execution of Mr Norris, apparently by underworld figures, was part of a pattern of deaths of police informers which is worrying chief constables.
In his case the police had withdrawn from a prosecution rather than reveal his identity as an informant, but senior detectives believe that the abandoning of the prosecution for drugs trafficking was the signal needed, which confirmed Mr Norris's position and led to his death.
His murder was the fourth in the past two and a half years of Scotland Yard informants. In each case the decision to withdraw from a prosecution based on their evidence was made after judges ruled in favour of defence applications that police sources should be disclosed.
Three of the murders were of informants living under new identities abroad; Mr Norris was the only one still active.Reuse content