John Barlow will finish his jail term as a convicted double murderer rather than an innocent man, after the Privy Council in London rejected his last appeal bid.

Barlow's wife Angela said she was "absolutely flabbergasted" at the Privy Council decision.

"I am so angry right now. There's no evidence to say that John has committed these crimes. He should have been freed. The whole justice system is a joke.

"I know absolutely and categorically that John did not do it. It is absolutely appalling what this country has done to our family and John, " Mrs Barlow told the Dominion Post.

The five law lords who heard Barlow's appeal said last night that while he had an arguable case, on the evidence he was properly convicted by the jury.

"The Board accordingly concludes...that, while the introduction of the misleading evidence...was indeed a miscarriage, no substantial miscarriage of justice actually occurred," the judgement said.

Though flawed evidence was used in his trial, the board found the "circumstantial case against Mr Barlow...overwhelming".

"While all the points made on behalf of the Crown were significant, their Lordships are particularly struck by certain very obvious aspects of the circumstantial case against Mr Barlow."

After twice going through trials that ended in hung juries, Barlow was convicted in 1995.

He was sentenced to 14 years jail for murdering Wellington father and son Eugene and Gene Thomas in 1994.

The appeal, by his lawyer Greg King hinged on faulty evidence given by FBI agent Charles Peters about tests on bullets thought to have been used in the murder.

Mr King told the law lords during the February appeal, that crucial evidence relating to the weapon and bullets that killed the men had been falsely linked to Barlow, a former antiques dealer, who is serving his time in Upper Hutt's Rimutaka Prison.

The FBI tests have since been discredited worldwide for providing a high number of false matches.

The Privy Council judgement pointed to evidence that Barlow was in the room about the same time the murders were committed; and the day after the murders, Barlow threw away his gun at the rubbish tip.

They also said before disposing of the gun, Barlow had substituted another barrel for the original barrel.

Lastly, the lords said Barlow had given three "contradictory and inconsistent accounts of his conduct" to the police and another person.

"These are merely four very broad aspects of the circumstantial evidence which stand out," the judgement said.

Mr King would not comment on the Privy Council's verdict last night. He told NZPA he would make a comment after reading the full court ruling.

Barlow was denied parole in April, after his November hearing was adjourned to allow for further psychological testing.

The board said that testing indicated he would still pose a risk to the community.

"We are not prepared to take the risk of releasing him at this time," the board's decision read.

However, it did support reintegrative leave for Barlow, saying he was clearly in the reintegrative phase of his sentence.

Mrs Barlow told the newspaper her husband would be "heart-broken".

"But he won't be surprised because the whole judicial system has kicked us in the guts right from the start. I don't know who is pulling the string here but it's a shocking decision."

Mrs Barlow believed her husband would be freed in March because he "ticked all the boxes".