As many as 1,000 prison staff in England and Wales are corrupt and use their positions to smuggle mobile phones and drugs to inmates, according to report.
The study, by the prison service's own anti-corruption unit and London's Metropolitan Police, said another 500 staff had inappropriate relationships with prisoners.
Several unnamed prison governors are quoted in the report, one of whom said there were far too many mobile phones in jail for them all to have been brought in without the aid of staff.
The prison service, which employs over 45,000 staff in England and Wales, said the problem was not widespread.
"We don't tolerate any form of corruption, we are determined to root it out," a Home Office official said. "We are confident the vast majority of prison staff are honest."
She said the joint study was the result of "extrapolation from anecdotal evidence and investigations".