French students murder in south-east London flat

French students Gabriel Ferez and Laurent Bonomo suffered hundreds of stab wounds in the attack by Sonnex, 23, and Nigel Farmer, 34.

Sonnex should have been in custody but was wrongly bailed.

David Scott said his profession risked becoming a "Cinderella" service in London because of officers' workloads.

He said he took "full responsibility for the performance of the staff that I led, and tendered my resignation as soon as it was clear that failings in the probation service were partly to blame for allowing the crimes to take place".

But he said it was "not solely the probation service that was at fault".

"I can only hope that every agency involved in public protection work will learn from the tragic failings that took place," he said in a statement.

The French research students were murdered at their rented south-east London flat in June 2008.

Sonnex had been bailed for handling stolen goods when he should have been remanded in custody.

His recall to prison was delayed and it took 33 days for the administrative process to be completed.

It was a further 16 days before Metropolitan Police officers went to Sonnex's house to arrest him.

Although Sonnex had tied up and threatened a couple he was given a verbal warning by his probation officer rather than being the subject of a review.

Mr Scott said London faced "unique challenges" in delivering a quality service for London.

And he admitted that "many more dangerous criminals are released onto the streets of the capital than anywhere else in the country".

There were, he said "recurrent problems in providing, supporting and equipping a sufficient number of skilled and experienced staff to monitor them".

Sonnex was told he will serve a minimum of 40 years. Farmer was told he must stay in prison for a minimum of 35 years.

The students' parents have pledged to sue the British authorities for a "failure" to prevent Sonnex carrying out his attack.

Guy Bonomo, Laurent's father, said the two students "would be alive today if the British justice system had not failed us".

Justice Secretary Jack Straw admitted Sonnex should have been in custody, saying: "It was the consequence of very serious failures across the criminal justice system that he had not been arrested and incarcerated some weeks before."

Mr Straw added that the fundamental problems within the probation service had been managerial claiming that the London operation had "under spent by £3.5m on a budget of £154m".