Inmates at a jail in Teesside have managed to smuggle dozens of mobile phones into their cells.
A BBC Tees investigation found that prison staff seized 79 mobiles at Stockton's Holme House last year.
The National Offender Management Service (NOMS) said the number of phones found reflected staff's success in finding them.
Holme House staff also seized small amounts of illegal drugs and home-brewed alcohol.
Geoff Dobson, deputy director of the Prison Reform Trust, which campaigns for major change in the penal system, blamed overcrowded prisons and the difficulties in policing them.
He said: "This is not particular to Holme House, it is across much of the estate. And it does allow for intimidating witnesses, organising criminal activities from inside prison and arranging drug drops.
"It is very difficult in an over-crowded prison system. I think that is much of the problem with large local jails, it is very hard to police them adequately."
He said Holme House did get a good report from the Prison Inspectorate last year.
Colin Moses, national chairman of the Prison Officers' Association, said there should be a way to block mobile phones from working within prison buildings.
He said: "There isn't a will from NOMS to block these phones, and we have the technology to do so."
He said mobile phones and drugs within prisons were linked. He said drug dealers used phones, and the drugs were used as currency for the phone.
Mr Moses congratulated staff for their vigilance in finding the number of phones they had so far, and called for extra security for prisons and more support for prison officers.
He added: "Anyone going into a prison who is carrying contraband should be prosecuted."
A statement from NOMS said there was a determination to tackle the issue of mobiles in jails, and that new legislation was planned to make it an offence - punishable with two years in jail - to possess a mobile or any parts in a prison.