One of the UK's largest prisons is replacing all its locks and keys over fears that inmates may have been given copies.
Police said a 40-year-old man was arrested at HMP Wandsworth in south London - which holds more than 1,600 prisoners - on suspicion of conspiracy to commit misconduct in a public office.
According to The Sun newspaper, he works for Government Facility Services Ltd, which provides services to 52 jails across the South of England.
The Ministry of Justice (MoJ) told Sky News that all the locks and keys at the category B jail are being replaced and the arrested man has been suspended.
The MoJ has not confirmed the number of locks and keys being changed or the cost of the move, but The Sun reported that some 3,200 keys are being swapped for an estimated bill of £1m.
A source told Sky News there was "currently no evidence" that the prison's security keys have been replicated or passed on to inmates.
A Prison Service spokeswoman said: "HMP Wandsworth has increased security at the gate and is operating extra identity checks as a result of a security incident.
"It would be inappropriate to comment further while a police investigation is ongoing."
The Metropolitan Police said officers attended HMP Wandsworth on 13 September and arrested a 40-year-old man on suspicion of conspiracy to commit misconduct in a public office "following a criminal allegation made to police earlier that day".
"He was taken to a west London police station and has subsequently been released under investigation pending further enquiries," a force spokeswoman added.
In 2006, Feltham Young Offenders' Institution had to replace all its locks and keys after security was compromised during a media visit.
ITN broadcast footage of a bunch of keys used at the jail in a report of a public inquiry into a racist killing.
The cost of changing the 11,000 locks and 3,200 keys at Feltham was thought to have to run into hundreds of thousands of pounds.