The number of prison officers in Erlestoke Prison,near Devizes, has increased significantly over the last year, as the establishment tackles rising levels of assault and self-harm.

But unions have warned of high turnover rates among prison staff, with inexperienced officers facing more violence and drug use among prisoners.

Newly-released figures from the Ministry of Justice show that there were 30 more officers in Erlestoke Prison in March this year than a year previously bringing the total up to 187.

Two years ago there was a riot in the prison which at the time was partly blamed on a lack of staff and inmates being kept locked up for long hours.

Glyn Travis, the assistant general secretary of the Prison Officers’ Association, said: “The biggest problem that we see now is that a lot of establishments have more than 50per cent of staff who have less than two years’ service in prisons.”

Figures released last month showed that there were 78 assaults in Erlestoke last year, more than double the level in 2012.