The study by the Prison Reform Trust (PRT) says the true figure may be even higher because the nationality of 1,200 people in custody is unknown.

There has been a 152% increase in numbers of foreign prisoners in the last 10 years, the report said.

In two prisons - the Verne, Dorset and Morton Hall, Lincolnshire - foreigners make up half or more of the population.

Juliet Lyon, PRT director, said: "Current failure to develop a strategy and respond humanely and effectively to foreign national prisoners condemns them to double punishment.

"Prison must be a very frightening and stressful place for those who are far from home, separated from their families, unable to understand basic instructions and rules or to ask for help and guidance."