Labour has failed to prevent another overcrowding crisis behind bars and must take “decisive” action to prevent more dangerous criminals being released, critics have declared. Shadow prisons minister Kieran Mullan said many of the 12,000 foreign criminals languishing in jails should be deported.

New Ministry of Justice figures on Monday revealed there were just 625 empty cells in the entire adult male estate. The prison population has ballooned to 84,088, with a capacity of 84,713. Further emergency measures to relieve the overcrowding crisis are not believed to be imminent. But justice chiefs are closely monitoring the situation.

Mr Mullan told the Daily Express: “Weeks into a Labour Government, the Justice Secretary decided to release thousands of dangerous criminals onto our streets.

“And for what? They have failed.

“Labour must now act decisively to tackle the remand backlog and deport the thousands of foreign national offenders in order to free up prison capacity so that those that have committed a crime are put behind bars.”

Ministers are relying on sentencing reform to prevent prisons from being overwhelmed.

At the height of the crisis last year, there were just 80 cells left, forcing justice chiefs to use police cells.

Under Labour’s plans, fewer criminals will be jailed and will instead be given community sentences.

Justice Secretary Shabana Mahmood has admitted criminals serving long prison sentences could be let out early under Labour’s sentencing overhaul.

Ms Mahmood backed Labour’s sentencing tsar, former Tory minister David Gauke, when he said dangerous offenders “will have to be released sooner if we are serious about not exceeding capacity”.

This could mean violent inmates, such as those convicted of manslaughter, will get out of prison early.

Mr Gauke also admitted the changes would be “controversial” and “difficult”.

And Ms Mahmood said ministers cannot prevent a repeat of the prison overcrowding crisis by only releasing more criminals on “shorter sentences”.

Speaking as she opened a new prison in North Yorkshire, Ms Mahmood said: “I’m not going to get ahead of the formal final recommendations of the sentencing review itself.

“I’ll wait until I get those and then I’ll make my decisions about which of the recommendations I want to pursue.

“But one thing is clear. We are already running back over 99% capacity in the prison estate.

“So, we're not going to get to a position of sustainable prison capacity, including with this new build as we're seeing now, with simply just action on shorter sentences.

“It's for the review to obviously look at the details as to how that might work and then ultimately it'll be for me to consider which of those recommendations I want to take forward.

"But short sentence reform on its own isn't going to be enough.

“We’re already over 99% capacity. We are filling prison spaces as fast as we can build them, and we know already we cannot build our way out of this crisis, even though we are also building at the same time.”