Police have been criticised for publicly appealing for help in tracing a convicted murderer - who has links to London and the Midlands - four years after he went on the run from an open prison.

Robert Donovan, 57, failed to return to Category D HMP Ford near Arundel in West Sussex after leaving on day release on June 19, 2010.

It has taken Sussex Police up until now to publicly appeal to help find Donovan, who was jailed in 1974 for stabbing a man to death in a West End street.

Donovan was aged 17 when theatre manager Edwin Thornley was robbed and fatally stabbed in the neck on Hungerford Bridge, the Daily Mail reported.

Sussex Police confirmed that this week's appeal was the first time the public had been made aware that Donovan was at large.

Conservative backbencher Philip Davies criticised the four-year delay by police in appealing for Donovan's whereabouts The MP for Shipley in West Yorkshire said: "I find it absolutely extraordinary that it has taken four years to make the public aware.

"On every possible level, the public should know straightaway for their own safety, they should know so they might be able to spot this person and alert the authorities, and they should know because the public has a right to know how successful our open prison regime is.

"I understand that the police were following other leads. Blimey, they were following them pretty slowly if it has taken four years to make a public appeal."

Steve Gillan, general secretary of the Prison Officers' Association (POA), said he found the delay in the police appeal "astounding".

He said: "Whilst we recognise that open prisons have an absolute value, we also believe that people have been rushed through because of the crisis in our prisons.

"If it's accurate that he absconded four years ago and it's only now that an appeal has been made, I find that astounding, to be honest."

Police urged the public not to approach Donovan, who may be using the alias David Green and has links to London and the Midlands, and may have family living in Portsmouth.

Although officers said there is nothing to suggest he poses a danger to the public, they advised anyone who spots him to dial 999 immediately.

Pc Steve Reed, of Sussex Police, said: "We have been looking for Donovan for several years without success which suggests he has left Sussex and could be elsewhere in the country.

"We need to speak to anyone who could have any idea where he is or where he could be.

"There is nothing to suggest that Donovan is currently a danger to the public but he has not served his sentence so we want to get him back into custody so justice can be done."

The disappearance of Donovan is the latest in a series of high-profile cases involving inmates who have gone on the run from open prisons.

Last month the same force, Sussex Police, appealed for help in trying to track violent robber Simon Rhodes-Butler, 37.

He was jailed for seven years in 2011 for a robbery on a garage owner in London, and vanished from HMP Ford on May 22 this year.

And the appeal to find Donovan follows the case of Michael Wheatley, 55, dubbed the Skull Cracker.

He was jailed for life for a raid on a building society while on the run from HMP Standford Hill in Kent, the second time he has been jailed for holding up the same branch.

His disappearance prompted ministers to launch a major review of the case, including a broader assessment of the release on temporary licence (ROTL) process.

Most recent Ministry of Justice figures show there were 1,242 indeterminate sentenced prisoners - that is, those serving life and imprisonment for public protection sentences (IPP) - as at December 31 last year. This includes 643 lifers and 599 IPP inmates in open prisons.

The Ministry of Justice had no comment on the four-year delay in appealing for Donovan's whereabouts, saying it was an issue for Sussex Police.