The Prime Minister has said he is “beyond frustrated” with the prisons overcrowding crisis as the new Labour Government prepares to free more criminals early.

Describing how the scale of the problem was “worse than I thought”, Sir Keir Starmer expressed his anger at being faced with taking emergency measures so soon in his premiership, accusing the former Conservative government of “total” and “shocking” failure.

He is expected to authorise a move which could see criminals automatically freed after serving 40% of their sentence, with an announcement measures to ease pressure on jail space anticipated on Friday – as the latest prison population figures are published.

Newly appointed Justice Secretary Shabana Mahmood is said to have been told the move could stop prisons running out of space within weeks.

Prime Minister Sir Keir Starmer is expected to authorise emergency measures to ease prison overcrowding this week (Alastair Grant/PA)

Speaking at the Nato summit in Washington, Sir Keir told LBC: “I can’t tell you how shocked I am to be in this position. It’s a basic function of government to have enough prison places for those that the courts are sending to prison.

“And that function was a complete failure under the last government, which they didn’t deal with, they didn’t address.

“It’s worse than I thought it was.”

“We’ll have to immediately put in place measures and then talk about the longer term strategy,” he said, adding: “But I’m beyond frustrated to have been put in this position so soon into a new administration, for a problem the last government knew was there, they knew they weren’t addressing it, they were simply leaving it for somebody else to pick up the pieces. That is not good enough.”

Speaking to the BBC, he said: “This is a total failure of the last government – and it doesn’t really matter what political party you support – to have left a situation where there are simply not enough prison places for the number of prisoners, to not grip that, but to just let it get worse and worse and worse, it is a shocking failure of government.”

He told Channel 4 the situation showed a “very high level of recklessness and irresponsibility” by the previous administration.

Sir Keir stressed there will be “exemptions” in place for high-risk prisoners when asked if he can guarantee dangerous inmates will not be allowed out on the streets.

Facing questions about longer term strategies to solve the problem, he told Sky News this is something which will need to be considered next because “we can’t build a prison overnight”.

Prison governors and union leaders have been warning for months that jails could reach capacity if no further action is taken.

The Prison Governors’ Association (PGA) said it “welcomes the speed with which this new administration is moving to deal with the prison capacity crisis” but warned: “It goes without saying that these changes will not reduce future demand on our system.

“We hope that over the coming months that a significant reduction in the prison population will bring some much-needed stability across the system.”

The body called for a “full review” of the crisis, adding: “The public must never be placed in this position again.”

Former Conservative justice secretary Alex Chalk reportedly pressed Rishi Sunak to change the rules but is said to have been repeatedly overruled, with the then prime minister fearing the move could be politically toxic and numbers could be managed with existing measures.

As he urged Labour to go ahead with the plan, Mr Chalk told The Today Podcast, on BBC Sounds, the move “will buy you 18 months, but it won’t buy you any more than that. You have to, as the new Justice Secretary, be very frank and credible about the long term”.

Alex Chalk was the previous justice secretary (Stefan Rousseau/PA)

As of Friday last week, Government figures show the adult prison population in England and Wales stood at 87,453 with a “usable operational capacity” of 88,864, indicating 1,411 spaces are available.

This gives an idea of how much free cell space is in men’s and women’s prisons.

Officials try to always keep a number of cells free as a contingency measure, so prisons have the capacity to operate safely and respond to any urgent or unforeseen circumstances.

The previous government expanded measures by which some inmates could be released from jail up to 70 days early, in a bid to free up cells, but concerns were raised that dangerous criminals could end up being eligible despite officials insisting offenders would continue to be supervised under strict conditions.

The Ministry of Justice is already building six new prisons to create an extra 20,000 places as demand grows for cell spaces, partially because of the Government’s campaign to hire 20,000 more police officers.

About 6,000 spaces have been created already and about 10,000 will be built by the end of 2025.

Whether the programme continues as planned since the change under Labour remains to be seen.