Hundreds of thousands of taxpayers’ cash is being squandered at Pentonville Prison because of delays to cases of foreign criminals facing the boot from Britain.

The number of criminals remanded at the Holloway jail for longer than necessary over deportation delays has doubled in the last year, according to a fresh report.

Numbers of foreign criminals have also soared 14 per cent over the past two years – to more than 400 of the Holloway prison’s approximately 1,200 population. Romanians are now the largest group of foreign criminals – accounting for 12.5 per cent.

The latest annual report, by the Independent Monitoring Board (IMB), also paints a “grim” picture of a prison in which inmates regularly self-harm, where one in 10 take drugs and prisoners vulnerable to attack – such as sex offenders – were left with vermin-infested accommodation.

IMB chairman Gordon Cropper, 66, is calling for Justice Secretary Chris Grayling to “urgently” invest in the prison.

He said: “It really is a very dismal physical environment. It needs capital investment as a matter of urgency.

“It’s simple human decency. You have two people in a very small, confined cell, they have to eat together, they will spend their sleeping hours together, they will have to use the toilet with very little privacy.

“It’s not a decent environment. No one is suggesting they should live in five-star luxury, but in a civilised nation they should be treated in a decent way.”

Delays in deportation cases is a result of a “reduction” in UK Border Agency (UKBA) involvement, the report suggests, which has been mired by savage cuts.

The report says: “It is the role of the UKBA to resolve this problem but its representation inside the prison has been reduced for much of the year and recently the vacancy has been filled by two part-time and inexperienced administrators.”

It adds it is “uneconomic” to keep prisoners facing deportation behind bars for anything up to more than a year, with it costing about £40,000 a year to keep each criminal jailed. Some prisoners are only kept for a few months, but overall it means the bill is in the hundreds of thousands.

The report revealed 474 reported cases of self-harm last year – up 21 per cent, while 10 per cent of prisoners tested positive for drugs.

“There are still too few opportunities for purposeful activity”, the report adds, while there is a lack of mental health training for staff.

Complaints between 2008/09 and 2011/12 increased 67 per cent while vulnerable prisoners on G wing were forced to put up with cockroaches, mice and “offensive smells”.

However, overall violence at the prison is down 10 per cent, “real progress” has been made in equality issues and the library is “a centre of innovative good practice”.

A Prison Service spokesman said: “The report by the Independent Monitoring Board at HMP Pentonville will be fully considered by ministers. They will respond in due course.”

Earlier this year a report by the Policy Exchange think tank proposed closing Pentonville and Holloway Prisons, as well as 28 others across the UK, and replacing them with a dozen 2,500-capacity “super prisons”.

A UKBA spokesperson said: “An increase in the number of immigration detainees at HMP Pentonville is not due to staffing issues. Immigration enforcement officers attend the prison five days a week.”