More than three quarters of prisons are suffering from worsening overcrowding, a new report has revealed.

A total of 77% of prison Boards of Visitors (BoVs) said their institution had been adversely affected by spiralling numbers of inmates.

The Prison Reform Trust is warning the situation is putting security and safety at risk - but the Prison Service describes the report as "melodramatic".

Some prisons had doubled up in cells meant for one

The group compiled a report based on 100 BoV visits and results show suicides have become more frequent and tension has reached "potentially explosive" levels.

The survey - the first of its kind - found 41% of respondents believed overcrowding was affecting the quality of the prison regime, such as the amount of work done to prevent re-offending.

More than a quarter - 27% - said prison security or prisoners' safety had suffered.

The report also disclosed:

• Overcrowding had led to the number of assaults increasing in some jails
• Inmates had to sit on the toilet to eat meals because the cells are so cramped
• A rise in drug taking in one prison as a result
• A prison with two people in 35 of its cells designed just for one
• Many jails saying overcrowding was putting an "intolerable burden" on staff

However, Martin Narey, director general of the Prison Service, said the report was "somewhat melodramatic".

He denied jails were in crisis, but added: "Overcrowding poses a serious challenge for the Prison Service.

'No crisis'

"While overcrowding has not threatened security, where our performance is better than ever, it makes it more difficult to get sufficient numbers of prisoners educational qualifications, drug treatment and other resettlement activities.

"I take overcrowding very seriously and regret very much doubling up prisoners in cells meant for one.

"However the PRT report is somewhat melodramatic."

He added: "A prison service which is producing a record number of educational qualifications and getting increasing numbers of prisoners into jobs on release is not service in crisis, as the PRT report suggests."

In August, the Howard League for Penal Reform warned that nearly two-thirds of Britain's prisoners were being held in overcrowded jails.

Worst was HMP Preston with 661 prisoners - but only 356 places.

The Prison Reform Trust is an independent charity, established in 1981.