Sir Keir Starmer has said he was “angry” to learn that offenders were being greeted with champagne outside of prison gates after being released early to ease overcrowding.

The prime minister said the government had been “forced” to let some offenders out early because prisons across England and Wales had become “so full” that police had been unable to make arrests.

Asked how he felt to see the reported images of prisoners celebrating, the prime minister told reporters in Rome: “I’ve spent five years prosecuting and putting people in prison. And being forced to release people who should be in prison makes me angry.”

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Last week, 1,750 prisoners were let out of prison to alleviate overcrowding in the system – something Sir Keir said the government had “no choice” but to do.

The latest government figures show there was a prison population of 86,333 as of Friday 13 September – down from 88,521 the week before, with more than 3,000 free prison spaces on the estate.

The decision to let out some offenders early has been met with anger in some quarters after one prisoner was accused of assaulting a woman on the same day he was freed. He has been charged and recalled to prison.

The government had said terrorists and convicted sex offenders were not among those being released.

Despite the backlash, the prime minister has defended the government’s decision, saying there was either the choice between releasing people in the way the government had or not being able to arrest criminals and put them in jail.

“You can imagine the impact on law and order in this country if we’ve reached a place where the police could not make the necessary arrests day by day,” he said.

“And you’d be having some pretty hard questions for me if it got to that stage.”

Sir Keir said his predecessor, Rishi Sunak, had also been advised to implement the early release scheme the Tories were now criticising the government about, and accused him of “delaying” the decision until after the election.

“He delayed until after the election and made the situation worse, so angry is what I feel – as I did when I was in Cobra, literally having to plot how many prison places we had, in order to assess whether we could contain the disorder,” he said.

“No prime minister should be in that position.

“To be put into a position where it’s a choice because prisons are so overcrowded that it’s a release scheme the likes of which we had to go down, or a point where the police say we cannot carry out our basic functions, is not a position I should have been put in.”

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Alongside the early release scheme, the prime minister said Labour would also change planning laws in England and Wales to allow more prisons to be built more rapidly and that the government was “already (trying) to move forward some of the projects in play”.

The government has already taken controversial steps to alleviate overcrowding, including by allowing the early release of prisoners who have served 40% of their sentence.

Prisoners in England and Wales, apart from the most serious offenders, are usually released on licence after serving 50% of their sentence – but from this month, this will be reduced to 40%.