Metropolitan Police commissioner Bernard Hogan-Howe has appealed for information about the murder of a "courageous" officer to mark the 20th anniversary of his death.

The police chief spoke at a memorial service for Detective Constable Jim Morrison, who was killed outside the Indian High Commission in central London on December 13 1991.

The 26-year-old was off-duty waiting for his wife Victoria in Covent Garden when he saw a handbag thief and chased him. When he cornered the suspect, he was stabbed, and the thief was seen running off into the Strand. The case has never been solved.

Members of his family, his friends and former colleagues gathered at the spot where he was killed to remember the officer, who was based at Bow Street police station.

Mr Hogan-Howe, who paid tribute to Det Con Morrison during the service, said afterwards that police were launching a fresh appeal to find his killer.

He said: "Jim was incredibly courageous. He took on a person who'd snatched a bag, and he gave his life. Here we are, 20 years after, and we still haven't caught that person. There will be somebody out there who knows who it is.

"It's vital that we mark his memory by trying to find his killer, and anybody out there who knows who did it, please tell us."

He added: "Day in, day out, the police officers at the Met do some incredibly brave things. Only very rarely do police officers lose their lives."

The Rev Philip Majcher, who led the half-hour service, said Det Con Morrison's "courage and dedication is an inspiration" to others. He said the officer "died courageously in the course of duty as a Metropolitan Police officer protecting the weak and vulnerable from those who would prey upon them".

Det Con Morrison made more arrests in the area than any other serving police officer at the time, according to his colleagues. The renewed police appeal comes with a £20,000 reward for information leading to the arrest and prosecution of the person responsible for his death.