There are only 100 spaces left in male prisons across England and Wales, Sky News understands.
The figure, first reported by the Telegraph, is the closest the system has come to running out of places amid an ongoing crisis in the sector.
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Government sources said they had always expected a squeeze on spaces over the bank holiday weekend due to the number of large-scale events, such as festivals and football matches.
While courts can sit on a Bank Holiday Monday, prisoners are unable to be released until Tuesday, seeing numbers further stack up.
However, the stark figure shines a spotlight on the issue being faced by the new government in tackling the lack of spaces.
Labour has repeatedly accused the Conservatives of neglecting the justice system during their time in office, saying prisons were at risk of overflowing.
Earlier in the summer, after becoming prime minister, Sir Keir Starmer green lit plans for some prisoners to be let out after serving 40% of their sentence – rather than the usual 50% – which will come into force on 10 September.
In the wake of the Southport protests, the Ministry of Justice also activated Operation Early Dawn, allowing defendants waiting for a court appearance to be held in police cells for longer until space is available.
But the weekend’s events, including Notting Hill Carnival, have pushed capacity even closer to the maximum.
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Speaking on Tuesday, Sir Keir said: “I can’t tell you how shocked I was when I discovered the full extent of what [the Conservatives have] done with our prisons, and it’s going to take time to fix it.
“I can’t build a prison by Saturday. We will fix it, we’ve already taken the measures that are necessary to make sure we can get through the disorder.”
Referring to the summer riots, he added: “I shouldn’t be sitting in the COBRA room with a list of prison places across the country on a day-by-day basis, trying to work out how we deal with disorder.
“But that’s the position I was put in and it’s frankly not good enough.”
Since the beginning of 2023, male prisons in England and Wales have had an occupancy rate of more than 99% the majority of the time, Ministry of Justice figures show.
The previous Conservative government started an early release prisoner scheme last October due to issues with overcrowding, which allowed certain offenders to be freed up to 70 days before the end of their sentence.
A Ministry of Justice spokesperson said: “The new government inherited a justice system in crisis and has been forced into taking difficult but necessary action to ensure we can keep locking up dangerous criminals and protect the public.
“Staff across the whole criminal justice system are working incredibly hard and the government will continue to support them before the changes come into effect on 10 September.”