Jails becoming too full to accept new inmates would spark a breakdown in law and order on UK streets within days, senior police sources have said. Police cells would be filled with arrested suspects and convicted prisoners, the source claimed, a situation that could be exploited by looters and criminals. The comments come as Shabana Mahmood, the justice secretary, prepares this Friday to set out emergency measures to free up space in prisons across England and Wales. There are 700 places left in the adult male estate. And the Prison Service would be “in trouble” if that figure dipped below 300, sources said. For the system to operate smoothly and effectively, there needed to be a minimum of 1,425 places. With prisons too full to accept new inmates, police would run out of cell space, the police source said. “If those prisoners aren’t collected we very quickly will run out of space in police cells. So many forces we estimated would run out of space in about three days or less. “And clearly that is a situation that is completely intolerable, because you would get to the point where arresting officers wouldn’t have anywhere to take people they are arresting.” Such a situation could escalate quickly and could be exploited by criminals who knew that they could not be detained by the police, it was claimed. “That would very quickly become evident,” the police source said. “And of course, then that in itself may impact on behaviour of the public and particularly criminals. So that would be a very serious situation to get into and uncharted territory and we need to avoid that at all costs.” A prisons source said that emergency measures needed to be introduced to avoid a critical breakdown of the system by the end of August. “If nothing was done, I would be professionally very, very, worried by the August bank holiday,” the source said. The Ministry of Justice said: “If prisons were to run out of places, police cells would rapidly fill up, courts would be forced to delay cases and police unable to arrest dangerous criminals, putting the public at risk from unchecked criminality on our streets.” An announcement on an early-release scheme had to be made this week so that probation officers had sufficient time to prepare for thousands of freed inmates. “You can’t leave an announcement much later than tomorrow because if you do we’re not going to have sufficient time to do the checks before we get into a headroom problem,” the prisons source said. The Prison Service has been running at over 99% occupancy for nearly two years, the source claimed: “That puts huge pressure on our ability to reduce reoffending, to deliver purposeful activity, and obviously on our staff. It definitely exacerbates issues like safety in terms of violence, it makes the estate much more tricky for us to run.” A relatively minor problem, such as an outbreak of bedbugs, could bring about a prison crisis because the system was already stretched. “We’re very vulnerable to shocks in the estate,” said the source. “That can be big, high-profile shocks … like the 2011 civil disorder [following the killing of Mark Duggan], like a prison riot, or it could actually be much more mundane things like an outbreak of bedbugs that would require us to close the wing, and fumigate it for two or three weeks.” The main measure to be announced by Mahmood will be automatic release of prisoners on “standard determinate sentences” after they have served 40% of their sentence, government sources confirmed. The strategy raises questions for the probation service, which is already overstretched. It will also anger many victims of crime who will question why convicted criminals are allowed to avoid most of their sentences. Keir Starmer said he was “beyond frustrated” with the prisons overcrowding crisis, as the Labour government prepared to free more criminals early. Describing how the scale of the problem was “worse than thought”, the prime minister 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.