Overcrowding at Portland Prison is forcing prisoners to share cells, eating, sleeping and even using the toilet in spaces designed for one.

Figures from the Ministry of Justice show that 496 prisoners were crammed into just 463 spaces at the prison in July.

Campaigners say that the unchecked rise of the prison population is responsible for the huge increase in assaults on staff and other inmates, culminating in the Government taking over HMP Birmingham from its contractor, G4S, after a damning inspection report.

The Prison Service measures its own capacity in terms of Certified Normal Accommodation - the number of prisoners it says it can accommodate in the “good, decent standard of accommodation that the service aspires to provide all prisoners”.

However, with the majority of prisons overcrowded across England and Wales, it also has a separate measure called Operational Capacity.

This is the maximum number of prisoners the Prison Service says each institution can safely handle while maintaining control and security.

In July, Portland’s population stood at 94 per cent of the Operational Capacity.

Prisons contain a number of one and two-person cells. In overcrowded prisons, more inmates will be put in cells than they were originally designed to hold.

Annual figures, published by the Prison Service in July, show the extent of the problem. From April 2017 to March 2018, 11 per cent of prisoners in Portland Prison were in overcrowded cells, 53 inmates on average.

Prison Reform Trust director, Peter Dawson, said that the figures “undermine” the basics of a “decent prison system.”

He added: “Overcrowding isn’t simply a case of being forced to share a confined space for up to 23 hours a day where you must eat, sleep and go to the toilet.

“No government has succeeded in building its way out of overcrowding. So we need a fundamental rethink about who we send to prison and for how long.”

The figures comes after a report, released last month by the Independent Monitoring Board (IMB) raised serious concerns over the management and running of the prison.

In the report, IMB inspectors said that the cell conditions for many prisoners were ‘insanitary and inhumane’ due to the lack of ‘maintenance work’.

‘Years of neglect’ had led to the conditions, the chairman of the IMB suggested.

Three inmates at the prison also climbed onto the highest roof on Thursday, August 9 to express their concerns over what is understood to be their poor treatment and living conditions

They got onto the roof shortly after 2pm before coming down shortly after 9.30pm following police intervention.

Andrew Neilson, director of campaigns at the Howard League for Penal Reform, said: “Cramming more people into prisons than they were designed to hold is a recipe for violence, drug abuse and mental distress.

“Bold action is needed to reduce the number of people behind bars and ease the pressure on other prisons.”

In response, a spokesman for the Ministry of Justice said: “Prison numbers can fluctuate, which is why we have robust plans in place to ensure we always have enough prison places for those sent to us by the courts.

“We will always ensure there are enough cells across the prison estate, and manage this in a way that gives taxpayers the best possible value for money.

“We are investing £1.3 billion to build modern new establishments, with up to 10,000 new prison places and better education facilities.”