Belmarsh prison opened in 1991 and holds 880 inmates

The lapse in security in May 2003 was revealed in a report of an inspection into Belmarsh prison in south-east London.

Mobile phones are banned from the jail because inmates could use them to plan their escape or run criminal enterprises from inside.

Anne Owers, the chief inspector of prisons, recommended the jail's security department be restructured and that managers should supervise cell searches.

The inspectors noted: "One prisoner found in possession of the phone admitted that he had concealed it in his underpants during a strip search - searchers had failed to lift the upper clothing to reveal the extra pair of underpants where it was hidden."

Too few staff and too many prisoners meant Belmarsh was not preparing inmates for release and therefore wasn't preventing re-offending Ann Owers

The watchdog's report was critical of the overall standard of security at the unit.

It said: "The security department was not sufficiently co-ordinated to provide a cohesive approach to security across the establishment.

"There was little evidence of dynamic security and the routine security safeguards, such as searching, were not being carried out thoroughly."

Belmarsh prison, which opened in 1991, holds 880 inmates ranging from low-risk unsentenced teenagers to Category A inmates and great Train Robber Ronnie Biggs is currently in its healthcare unit.

Staff shortages

The report said that there had been some improvements but Ms Owers told Radio 4's Today programme that the problems were due to staff shortages.

"We found problems with co-ordination of security and we also found because of staff shortages, staff were being re-deployed elsewhere," she said.

"Too few staff and too many prisoners meant Belmarsh was not preparing inmates for release and therefore wasn't preventing re-offending."

She added: "It has always been a problem to recruit and maintain staff in South East England."

Terrorism detainees

The inspectors made 147 recommendations which included that remanded young adults aged 18 to 20 should no longer be held alongside serious, convicted adult inmates.

They also said the seven men interned under Home Secretary David Blunkett's Anti-Terrorism, Crime and Security Act - introduced shortly after the 11 September attacks - "should be provided with access, support and help that recognises the unique circumstances and indefinite nature of their detention".

Geoff Hughes, governor of Belmarsh, said security remained the highest priority, and questioned whether the inspection team had fully understood the way the unit was organised.

However, Juliet Lyon, director of the Prison Reform Trust, said: "Last year Belmarsh had the lowest level of purposeful activity of any prison, at just 11 hours a week per prisoner.

'Sharpening up'

"Now the chief inspector's report reveals significant failings, which begs the question as to why a complex high security jail should be holding hundreds of young petty offenders who could much more usefully be placed on enforced community service programmes."

The BBC's home affairs correspondent Danny Shaw told Radio 4's Today programme that the inspectors wanted a general "sharpening up" of security at the jail.

"This is not a huge security problem but you are talking about one of the nine top security prisons in the country with a special secure unit which holds exceptionally high-risk inmates.

"There are also terrorism detainees being held at Belmarsh as well, so security has to be a priority - which the report acknowledges it is - but it has to be absolutely number one without any flaws at all."