Double murderer Christopher Halliwell may have killed more women, senior detectives believe.

The killer taxi driver may die behind bars after being convicted of murdering 20-year-old prostitute Becky Godden - four years after he was jailed for the abduction and murder of office worker Sian O'Callaghan, 22.

Thirty years ago, Halliwell spoke of strangling a woman during sex and asked how many victims he would have to kill to become a serial killer.

Police urged the 52-year-old to reveal whether he has killed more women - as several are still missing from Swindon.

Being a taxi driver gave him the opportunity to cruise the streets late at night without suspicion, and as a keen fisherman, he knew the countryside outside Swindon.

Ex-Detective Superintendent Steve Fulcher, whose blundering five years ago breached strict police guidelines and saw Halliwell initially escape justice for Miss Godden's murder, said he had "caught a serial killer" and his "right and moral" actions prevented other women being murdered.

Mr Fulcher said: "Halliwell is an evil and depraved violator of women. I caught a serial killer; preventing any further girls being murdered."

Miss O'Callaghan's mother Elaine Pickford, 54, believes the former groundworker would have killed other women.

"I don't think a man started killing at the age he killed Becky and I don't think he'd have gone the length of time between Becky and Sian without needing his fix," she said.

Detective Superintendent Sean Memory, who led the new investigation into Halliwell, said the killer cabbie may have had other victims and appealed to him to come clean.

"I am really open-minded, there may be others - there is an eight-year gap between Becky and Sian," Mr Memory said.

"I would appeal to Christopher Halliwell, actually, if he wants to speak I'm willing to speak with him.

"I can't rule out that there are other victims, however I have no direct evidence at the moment to suggest there are."

Halliwell will be sentenced on Friday at Bristol Crown Court after a jury took less than three hours to convict him of murdering Miss Godden.

Trial judge Sir John Griffith Williams branded the father-of-three a "liar" and said he would receive either a whole life order or a "substantial" minimum term of imprisonment.

Halliwell, formerly of Ashbury Avenue, Swindon, laughed and smiled as the verdict was returned by the jury.

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Members of Miss Godden's family cheered, wept and hugged each other when they heard the guilty verdict.

As he was led to the cells, he paused to stare and smirk at them.

Halliwell, who represented himself during the two week trial, is already serving a life sentence for the murder of Miss O'Callaghan who he abducted in his taxi as she made her way home from a night out in Swindon in March 2011.

He confessed to her killing and took police to her body before offering "another one" and leading them to where he had buried Miss Godden eight years earlier.

A High Court judge later ruled Halliwell's admissions could not be used as Mr Fulcher had breached police guidelines on interviewing suspects.

The charge of murdering Miss Godden was withdrawn until March this year following an investigation by Wiltshire Police that uncovered overwhelming evidence against Halliwell.

But Sir John ruled the confession could be presented to the jury following two days of legal argument in July.

The judge also stated that the jury could be told of Halliwell's conviction for murdering Miss O'Callaghan, as well as telling a police doctor he had been arrested because he had "killed two people".

Becky Godden pictured when she was 15 in the late 1990s. Her mother never gave up hope that she would return home alive until her body was discovered nine years after she went missing (PA)

During the trial, prosecutors highlighted the similarities between the two victims.

They were young, attractive with a slight build and disappeared from Swindon in the early hours of the morning.

Halliwell initially wrongly believed Miss O'Callaghan was a sex worker, while Miss Godden was a prostitute. He admitted having sex with Miss Godden before she died and prosecutors insist that Miss O'Callaghan's murder was sexually-motivated.

Both women were strangled and their bodies were deposited in rural locations close to each other.

Miss Godden was buried naked in a shallow grave while Miss O'Callaghan was found partially clothed - with Halliwell later admitting he had cut her underwear and leggings in an attempt to remove them.

Halliwell described himself as a "sick f*****" before telling Mr Fulcher that he had strangled Miss Godden after having sex with her.

He led the experienced detective to Oxo Bottom field in Eastleach, Gloucestershire, where he fumbled in a wall for a dip then paced heel to toe to the exact spot where she was buried eight inches below.

But Halliwell refused to cooperate with officers after being taken to a police station - later claiming this was an act of revenge due to his "loathing" of Mr Fulcher.

After Halliwell confessed to murdering the two women and took Mr Fulcher to their bodies, the detective announced live on television that the remains of Miss O'Callaghan had been found at Uffington, Oxfordshire and the location of a second body had been identified to him.

But he did so before a solicitor had arrived at the police station for Halliwell.

The detective resigned from Wiltshire Police in May 2014, months after being handed a final written warning when he was found guilty of gross misconduct following a formal conduct hearing.

Miss Godden's mother Karen Edwards backed Mr Fulcher's actions, saying she would always be grateful for "bringing my little girl home".

"I will also respect him and will be indebted to him for making that moral decision as a police officer but he should have never have suffered the terrible consequences, loss of reputation and career for doing such a thing," she said.