Government plans to radically reform sentencing will lead to an increase in crime by as much as 6% in a single year, according to police chiefs.

The reforms, which cover England and Wales, involve a presumption against short sentences of a year or less, with community sentences used instead, and those jailed being released earlier than currently the case.

The hope, which policing bosses said they share, is that offenders will experience greater efforts at rehabilitation, and in the medium to long term the changes will cut offending.

But Jason Devonport, the National Police Chiefs’ Council lead for criminal justice reform, warned of an increase in recorded crime of 4 to 6% across England and Wales in the first year after the changes were enacted.

That is equivalent to tens of thousands of additional crimes. In the year to June 2025, 6.6 million crimes were recorded in England and Wales, official figures say.

Devonport said: “We are expecting that, whilst the programmes in the community are being ramped up by the probation service as part of the implementation plan to support offenders to rehabilitate, we expect, certainly in the short term, there will be an increase of offending.”

Gavin Stephens, the NPCC chair, said: “The ambition on this from everybody is that over the medium to long term, if the new approach to rehabilitation is right, it should drive it down. But there’s no doubt, in the short term, we’re working on an assumption that there’s going to be an increase.”

Some convicted of domestic violence or sex offences may be included in those realised from prison early or not jailed at all.

More probation officers, about 1,500, are being recruited and need training to give the reforms a better chance of working.

Police say they are pressing government for £300m to £400m extra for the increased demands, such as greater monitoring of offenders in the community.

It is an unprecedented intervention by police chiefs, directly pinning their forecast of a significant rise in crime on a government policy.

Devonport, who spent 18 months as a prison governor, said: “I do believe in the sentencing bill and I believe in rehabilitation, but it has to be properly funded.”

Stephens said: “We’ve all been in policing long enough to know that some of the things that help people stop offending or desist from offending are not going to be resolved by short sentences in particular.

“So that’s a fundamental reason why we’re supportive of this.”

Ellie Butt, of Refuge, said her organisation was deeply concerned: “The risks posed by domestic abuse perpetrators cannot be underestimated. With the sentencing bill set to create a presumption that custodial sentences of less than 12 months will be suspended, it is crucial that safeguards exempting domestic abuse offenders are consistently applied.

“Survivors’ confidence in the criminal justice system is already at breaking point, and many tell us they receive inadequate responses from police when they report abuse. The government cannot afford to take decisions that will reduce the police’s ability to effectively respond to domestic abuse.”

Prisons in England and Wales are so overcrowded the new Labour government last year implemented a policy of early release for convicted offenders.

If there is an increase in crime, that could pose political danger for Labour.

The shadow home secretary, Chris Philp, said: “This revelation proves what common sense tells us: Labour’s weak and reckless approach to sentencing means many more criminals will be out on the streets, where they will commit more crime.

“The police are telling us Labour’s policy will make us less safe, and the government must change course.”

By cutting reoffending through rehabilitation, the long-term hope has been that crime will fall along with the cost of repeatedly having to lock up the same offenders.

But measures such as electronic tagging of offenders have had limited success. One police source said: “Lots of faith is being placed in tags. There is a growing consensus in policing that tags are the emperor’s new clothes. In the real world they don’t work perfectly. There is a fair degree of failure and non-compliance.”

A House of Lords report published on Thursday said the use of tagging is likely to double under the sentencing bill, and warns it may fail.

The report from the Lords’ justice and home affairs committee says more money is needed for the probation service, and the government lacks a clear strategy about when electronic monitoring should be used.

The Ministry of Justice, the department responsible for the changes, has been approached for comment.