The “shocking” prisons crisis is even worse than feared, Keir Starmer has said as the government prepares to release tens of thousands of inmates early in a bid to prevent jails becoming full. The prime minister suggested he was opposed to freeing violent criminals and sex offenders when ministers announce the terms of a new prisoner release scheme for England and Wales on Friday. The Guardian understands that ministers are examining whether the scheme can exclude domestic abusers. It is expected to allow early release for those who have served 40% of their sentence, instead of 50% under current rules for inmates serving determinate sentences. Whitehall sources have said this could lead to the release of more than 20,000 inmates over many months. The former Conservative justice secretary Alex Chalk is understood to have pushed the former prime minister Rishi Sunak for months to change those rules, but Sunak is said by insiders to have repeatedly overruled Chalk, believing that it would be politically toxic and that numbers could be managed within existing systems. One figure from the previous administration said: “We held our nerve on this for months and found other ways to keep the system operational and get new prison places online – Labour have bottled it.” Speaking to reporters on a trip to the Nato summit in the US, Starmer said that Conservative government had been reckless in letting prisons come within a fortnight of reaching overflow. “It is shocking for our country to have got into a state where we have too many prisoners and not enough prison places,” he said. “To a point where any government is now in a position where it has to release prisoners early. That is a shocking indictment. That is a total failure of government.” Prisons are understood to have been operating at 99% capacity for 18 months with 100 net prisoners being added every week. A government source said: “What we encountered when we came in was a shocking act of self-harm to the country. They had pulled small levers to try and delay prisons reaching capacity by weeks, but had never taken the tough decisions. There are now no good options.” A decision must be made this week to prevent full capacity being hit by 1 August. The justice secretary, Shabana Mahmood, will make a statement on Friday about the terms of the prisoners’ release. Starmer pointed to a report in May which found that a high-risk prisoner had his release date brought forward despite a history of stalking and domestic abuse, and being subject to a restraining order. Asked what the terms of the prisoner release scheme would be, Starmer said he had “pressed Rishi Sunak hard” on that case in parliament, adding: “We will set that all out when we make a statement in coming days. But you know from my question to Mr Sunak … where I stand on this.” Starmer said: “Some of what we’ve found is shocking … The situation is worse than I thought it was. I’m pretty shocked that it’s been allowed to get into that situation. “It’s reckless to allow them to get into that place … We’re going to have to set out the state of affairs; what we intend to do about it, but it will include this question for risk categories.” Labour’s manifesto promised a tough line on domestic abuse and pledged to halve violence against women and girls within a decade, including dedicated “rape courts” and domestic violence experts taking 999 calls. On Wednesday a Home Office adviser warned that the pledge would put more pressure on prisons, with an additional 10,000 domestic abuse convictions if just one in three victims came forward. Prisons are understood to have been operating at 99% capacity for 18 months with 100 net prisoners being added every week. A government source said: “What we encountered when we came in was a shocking act of self-harm to the country. They had pulled small levers to try and delay prisons reaching capacity by weeks, but had never taken the tough decisions. There are now no good options.” A decision must be made this week to prevent full capacity being hit by 1 August. The justice secretary, Shabana Mahmood, will make a statement on Friday about the terms of the prisoners’ release. Starmer pointed to a report in May which found that a high-risk prisoner had his release date brought forward despite a history of stalking and domestic abuse, and being subject to a restraining order. Asked what the terms of the prisoner release scheme would be, Starmer said he had “pressed Rishi Sunak hard” on that case in parliament, adding: “We will set that all out when we make a statement in coming days. But you know from my question to Mr Sunak … where I stand on this.” Starmer said: “Some of what we’ve found is shocking … The situation is worse than I thought it was. I’m pretty shocked that it’s been allowed to get into that situation. “It’s reckless to allow them to get into that place … We’re going to have to set out the state of affairs; what we intend to do about it, but it will include this question for risk categories.” Labour’s manifesto promised a tough line on domestic abuse and pledged to halve violence against women and girls within a decade, including dedicated “rape courts” and domestic violence experts taking 999 calls. On Wednesday a Home Office adviser warned that the pledge would put more pressure on prisons, with an additional 10,000 domestic abuse convictions if just one in three victims came forward. Nicole Jacobs, the government’s domestic abuse commissioner, wrote to Starmer to highlight that the current prisons crisis was taking place at a time of historically low rates of convictions for those perpetrating domestic abuse. Other experts warned that the early release system was a short-term fix and the overcrowding crisis would need significant investment in the coming years. Tom Wheatley, the president of the Prison Governors Association, said Starmer appeared to have heeded warnings for emergency action, but warned that investment would also have to be found to fix a broken system. “We’re hopeful that the penal populism of the last government is a thing of the past and that the new government is willing to invest in our service so that we can effectively protect the public by reducing the likelihood of reoffending,” he said. Mark Fairhurst, the national chair of the Prison Officers’ Association, said Starmer’s promise to act followed years of neglect by Tory governments. “By the end of July, prisons will be full,” he said. “So no matter what people’s opinions are, or how unpalatable the announcement may be, there is an urgent need to decrease the prison population and give our members some breathing space so we can start to reverse the chaos of the last 14 years.” Last week Starmer appointed James Timpson as his prisons minister. The owner of a successful chain of keycutters, Timpson has long employed ex-offenders and has argued the country is “addicted to punishment … locking people up far too long. And we’re sending people to prison when actually all the evidence suggests prison is not the right place for them.” A Ministry of Justice source said there were no plans to extend the early release scheme from 70 days early, as announced in May. It was previously extended to 60 days, from 18 days in March. The government is understood to have no plans for a queuing system for sentences and it is hoped that the releases will buy enough time to begin efforts to build capacity and start to address reoffending rates. Speaking to the Today Podcast, Chalk said the releases were “the right way to proceed” but said it would “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.” On his way to Washington for the Nato summit, Starmer also warned there would be no immediate fixes to the small boats crisis of asylum seekers crossing the Channel, suggesting he expected numbers to rise over the summer months. He said the government would not put any number on when they expected the crossings to begin to fall. “That unfortunately is what we’ve inherited – we can’t change that overnight … Nobody but nobody should be making these crossings. The numbers are going up, not down,” Starmer said.