A bounty of £20,000 has been offered to catch and convict the last remaining suspect wanted for the murder of a Bradford police officer.

Robbery mastermind Piran Ditta Khan, 50, fled the country after 38-year-old PC Sharon Beshenivsky was shot dead during a botched armed raid at the Universal Express travel agency in Morley Street, Bradford, four years ago today.

On the anniversary of the murdered mother-of-three’s death, detectives are asking people to help trace Khan, who was known as Uncle Pete and is believed to be in hiding in Pakistan.

They have released a wanted poster, through Crimestoppers, offering the £20,000 reward.

Detective Superintendent Andy Brennan, who has led the inquiry which has already seen three murderers jailed for a total of more than 100 years, has now vowed to bring Khan back to Britain to face justice.

He said: “I am as committed now as I was in November, 2005 to see this inquiry through to a conclusion. It is extremely important we trace Piran Ditta Khan and place him before the courts in Great Britain.”

And, in an appeal directly to people in Pakistan, he said: “If you know, or suspect you know, where Piran Ditta Khan might be, please tell us.”

Mustaf Jama, 29, who was jailed for PC Beshenivsky’s murder earlier this year, was the third of a gang described by a judge as “ruthless, dangerous men”, which included Jama’s brother Yusuf and ringleader Muzzakher Shah.

They were each jailed for a minimum of 35 years for PC Sharon’s murder.

In December, 2006, the victims of the Universal Express robbery said Piran Ditta Khan was living in the remote village of Kiri Afghanan in the Jhelum district of north Pakistan. At that time, Khan’s wife and six children lived in north London.

The wanted man was reported to be a martial arts expert who enjoyed kick boxing and worked as a doorman in clubs in Bradford during the 1980s.

A relative, who did not want to be named, spoke out the year after the murder because Khan had “brought shame” on the family.

This year the Prime Minister, Gordon Brown, unveiled a memorial stone in Bradford in honour of PC Beshenivsky, of Hainworth, near Keighley.

He stood alongside then Home Secretary Jacqui Smith, film director Michael Winner and the Beshenivsky family in a sea of flowers and messages at the spot where the police officer was gunned down and said the whole nation owed PC Beshenivsky a “debt of gratitude”.