Former colleagues of a Croydon policeman fatally stabbed during an armed robbery at a New Addington post office are to hold a memorial ceremony to mark the anniversary of his death.

Sergeant Derek Robertson was knifed to death by one of three robbers who held up the post office on February 8, 1994.

His widow Christine Wood will join current and former officers outside Addington police station next month to mark the 20th year since his death.

Mr Wood was one of four officers who responded to the robbery, in which three men broke into the office and laid in wait for the postmaster.

The robbers overpowered him when he arrived and held him at knife point while they waited for the timed locks on the office's safe to disengage.

Their plan went awry when the postmaster received a phonecall from his wife, who sensed trouble and rang the police.

Sergeant Robertson was one of the first officers to arrive and covered the rear exit of the post office. Soon after arriving, he relayed by radio to colleagues that the three robbers had left by the back door, one holding a knife to the postmaster's throat.

Officers caught and arrested two of the robbers but then found Sgt Roberton had been repeatedly stabbed trying to arrest the third. He died at the scene despite paramedics' attempts to save him.

Robert Eaves, the third robber, was found nearby and arrested following a short manhunt.

Eaves, then aged 32, was jailed for life in January 1995 following a trial at the Old Bailey. He will serve a minimum of 25 years.

Eaves's accomplices, aged 38 and 32, were jailed for 12 years for manslaughter.

All four officers who attended were awarded commendations for their bravery by Sir Paul Condon, then commissioner of the Metropolitan Police.

Sgt Robertson posthumously received a high commendation - the top honour than can be bestowed on an officer. He was also awarded the Queen's Gallantry Medal in 1995.

His colleagues who responded to the robbery with him are to attend the memorial ceremony on Sunday, February 9.