The justice secretary has apologised to the families of the two murdered French students for the ‘serious failures across the criminal justice system’ that left one of the defendants free to kill when he should have been incarcerated.

Dano Sonnex and Nigel farmer were found guilty at the Old Bailey of the murders of students Laurent Bonomo and Gabriel Ferez.

At the time of the murders Dano Sonnex was subject to a warrant for his arrest and recall to prison for breach of his probation licence conditions.

Jack Straw said: ‘On behalf of the government I express my deepest sympathies to the families of Mr Bonomo and Mr Ferez and I am ensuring they receive every possible support.’

‘These were the most appalling, sadistic and gratuitous murders. The direct responsibility for these killings must lie – as the jury found – with the criminals Sonnex and Farmer. But it is also the case that Sonnex could and should have been in custody at the time he committed these murders. It was the consequence of very serious failures across the criminal justice system that he had not been arrested and incarcerated some weeks before.’

Straw said he took full responsibility for the ‘unacceptable’ failings of the probation service. The chief officer of London Probation resigned and a new interim chief, Paul Wilson, was appointed to initiate changes. One hundred new probation officers will be recruited in London in the next two years

Straw said the independent review by the National Offender Management Service concluded the problems were managerial not budgetary. He said last year London Probation under spent by £3.5m on a budget of £154m.

‘It was a failure to allocate London Probation resources sensibly between the different areas, to supervise workloads and to heed obvious warning signs – like sickness rates in Lewisham and Greenwich probation area (the area that had responsibility for managing Sonnex) being an average of 27 days per year, more than twice the London Probation average.’

‘Whilst risk can never be eliminated entirely and the management of chaotic offenders is inherently difficult, there is clear evidence of poor judgement and failures to act promptly by the agencies responsible for managing Dano Sonnex,’ said Straw.

He said he would personally monitor the progress made by the probation service until he was satisfied that high standards were being achieved.