It is “deeply regrettable” that prisoners who were jailed under an old harassment law were wrongly released early, the Prime Minister has said.

Some 37 offenders were released in error as part of the Government’s early release scheme, as ministers battled to stop prisons becoming full.

The PA news agency understands some offenders, sentenced using previous harassment law, were not flagged as being exempt from the temporary early release scheme, which Lord Chancellor Shabana Mahmood introduced to “avert disaster” amid a capacity crisis.

Stalkers and domestic abusers were among those who were accidentally released.

Speaking to journalists during a visit to the United Nations in New York, Sir Keir Starmer said he was concerned that they had been wrongly released.

He said: “It’s deeply regrettable that they were released.

“As I understand it, to do with the classification under old legislation and I think all bar one are back in custody.

“But of course, I’m concerned, and we’ll ensure that all victims have the support that they need.”

Ms Mahmood cut temporarily the proportion of sentences which inmates must serve behind bars from 50% to 40% during the summer.

The Cabinet minister exempted some offenders from the scheme if their crimes were linked with stalking, controlling or coercive behaviours in an intimate or family relationship, non-fatal strangulation and suffocation, and breach of restraining order.

However, the technical glitch affected breach of a restraining order cases prosecuted using the Protection from Harassment Act 1997, after its replacement – the Sentencing Act – was passed in 2020.

Offenders prosecuted using the older legislation were not labelled as being ineligible for early release.

Among the 37 offenders mistakenly released, all but one have been returned to custody. Staff have been given guidance in an effort to block further incorrect releases.

“Public safety is our first priority,” the Ministry of Justice spokesman said.

“That is why we took decisive action to fix the broken prison system we inherited and keep the most dangerous offenders locked up. This included blocking the early release of domestic abuse offences such as stalking and controlling behaviour.

“We are working with the police to urgently return a very small number of offenders – who were charged incorrectly and sentenced under repealed legislation – to custody.

“The convictions remain valid with offenders monitored since their release and will soon be back behind bars.”

Liberal Democrat justice spokesman Josh Babarinde MP described the mistake as “deeply shocking news” and called on the Government to develop a “plan to reduce reoffending and tackle the prison backlog without delay”.

He said: “The public deserves immediate action so that all those who were mistakenly released from prison are swiftly returned, and to prevent this from happening again.

“The Conservatives have neglected our criminal justice system, and now the chaos of their actions is showing.”

Domestic abuse commissioner Nicole Jacobs said: “Anyone who breaches a restraining order is likely to be a high-risk individual who may be fixated on their victim, undeterred by legal restrictions that have been put in place.”

She added: “Probation should not rely on an individual’s index offence to determine risk but use local intelligence to understand if there is a history of abuse and exempt them from early release on that basis.

“I am encouraged that virtually all offenders released early appear to have been recalled, and efforts have been made to rectify the error so it cannot happen again.”