About 5,500 offenders will be released early from prison as part of a temporary scheme to “avert disaster”, the justice secretary has said. It is the first time the government has provided a figure for the number of prisoners who will be released in England and Wales as part of emergency measures to prevent the prison system from reaching the point of collapse. “It is now clear that by September of this year our prisons will overflow. There is only one way to avert disaster,” Shabana Mahmood told MPs on Thursday. Rishi Sunak had left “a timebomb ticking away” when he called an election, she said in a statement on the plans. The Guardian revealed on Tuesday that Sunak was warned by senior civil servants a week before he called the election that he was at risk of breaching his legal responsibilities if he failed to take action over the prison overcrowding crisis. Mahmood said: “If that bomb goes off, if our prisons run out of space, the courts would grind to a halt, suspects could not be held in custody and police officers would be unable to make arrests, leaving criminals free to act without consequence. In short, if we fail to act now we face the prospect of a total breakdown of law and order.” While most qualifying prisoners serving standard determinate sentences currently leave prison at the halfway point, she said the government had no option but to introduce a temporary change in the law. Prisoners who do not fall into exempt categories, such as those serving sentences for serious crimes, will be released under the new scheme after they have served 40% of their sentences, rather than 50%. She said the government’s impact assessment estimated approximately 5,500 prisoners would be released in September and October. The plans will be reviewed in 18 months’ time when the situation in prisons had “stabilised”, said Mahmood, who added that data would be published on a quarterly basis on the number of those released. A review into how the capacity crisis in prisons had been allowed to happen would also take place. “I understand some may feel worried but I can assure them that we are taking every possible precaution,” she said, citing exclusions in relation to those serving sentences for the most dangerous crimes, including sexual and serious violent offences. This will also apply to offences linked to domestic violence, including stalking. Responding for the opposition, the former prisons minister Edward Argar said his party recognised the challenges and pressures facing prisons and that “in government we took the right decisions” to significantly toughen up sentences. He asked Mahmood if domestic abusers convicted of common assault would be exempt from the new policy and what criteria she would commit to when it came to ending the measures. She replied that common offences connected to domestic abuse would be “caught” within the statutory instruments set up to introduce the release plan. Mahmood reiterated her claim last week that Sunak and his former ministers were “the guilty men” who should be held responsible for “the most disgraceful dereliction of duty” by failing to address the prisons crisis. Advice sent to the then prime minister on 15 May said that failing to make an urgent decision on prison capacity would mean the criminal justice system in England and Wales reaching the point of “critical failure”.