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

Both Dorset's Guy's Marsh Prison in Shaftesbury and Portland Prison are oversubscribed according to Ministry of Justice figures, as is HMP Winchester where many prisoners from Dorset are sent.

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.

The figures show that 491 prisoners were crammed into just 453 spaces at Guy's Marsh in March.

In HMP Portland, 489 prisoners were sharing 463 spaces.

At Winchester, 629 prisoners were sharing 458 spaces.

Campaigners say that the unchecked rise of the prison population is responsible for the huge increase in assaults on staff and other inmates – a situation described last week as a "national emergency".

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, they also have a separate measure called 'operational capacity' – the maximum number of prisoners each institution can safely handle while maintaining control and security.

In March, Guy's Marsh's population stood at 96 per cent of this capacity, Portland's at 92 per cent and Winchester's at 93 per cent.

Frances Crook, chief executive of the Howard League for Penal Reform, said last week: "This shameful rise in violence and self-injury is the direct result of policy decisions to allow the number of people behind bars to grow unchecked while starving prisons of resources."

Figures released last month showed 211 assaults were recorded at Guy's Marsh in 2017, more than triple the number in 2012.

There were 303 assaults at Portland in 2017, double the number in 2012, and 369 assaults at Winchester – six times the number five years ago.

Between the three prisons there were some 340 attacks on prison staff by inmates last year.

There were also 172 cases of self-harm recorded at Guy's Marsh last year, 188 at Portland and 474 at Winchester.

The director of the Prison Reform Trust, Peter Dawson, said: "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.

"It directly undermines all the basics of a decent prison system, including work, safety and rehabilitation.

"Despite a virtually permanent programme of prison building, overcrowding has been an unchanging reality of our prison system since 1994. Building prisons isn't the solution – breaking our addiction to imprisonment is."

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."