Police have revealed that 13 killers are among the 821 criminals known to be on the run in the UK, with some having eluded justice for decades.

One of them is Douglas Wakefield, convicted of two killings, who is now a woman after having a sex-change operation in jail.

Wakefield was convicted of stabbing his uncle dead in 1974, and in prison was found guilty of the attempted murder of a prison guard and the manslaughter of a fellow inmate.

In jail, he was granted permission to have a sex change operation, changing his name to Tai Pilley, and entered into a civil partnership with another female inmate.

Wakefield fled when released on licence in 2003.

Robert Hamilton, 60, who was sentenced to life for battering a gay man to death in 1970, has been on the run for a total of 30 years after absconding twice.

He escaped from Sudbury open prison in 1983 and went on the run for 17 years, before he was arrested again in the south of France.

He absconded again in 2003 when he was released from Wormwood Scrubs prison, south London, on licence, and has not been seen since.

Many of the killers fled when recalled to prison after breaking the terms of their release.

One of them is Brian Lynch, 46, who has been on the run for 12 years after absconding while released on licence from Kirkham prison, Lancashire.

One of the most recent cases is Paul Flint, 48, who absconded for the second time in just over a year in July. He had been serving time in Ford open prison, West Sussex.

He was convicted of a murder committed during a bungled burglary in 1990 and is believed to be responsible for a string of burglaries since.

Among the 821 on the run are convicted rapists and violent criminals.

Javed Khan, head of the charity Victim Support, told the Sunday Times: "Examples like this risk damaging public confidence in the ability of the criminal justice system to keep them safe."

Convicts are often released on licence on condition that they regularly contact their probation officers. Killers must abide by these conditions for life.

Recently, prisons minister Jeremy Wright revealed that 38 murders were committed by criminals released on licence between 2008 and 2011.

A Prison Service spokesman said: "In more than 99% of cases where an offender has been recalled, the individual has been successfully returned to custody.

"Where an offender flees the country and their whereabouts are known, the UK does everything it can to return them to custody."