Prisons are a crucible for corruption, a former governor claims. Staff are working in the toughest conditions the system has seen in decades. Thousands of experienced staff have left and some areas are struggling to replace them. Morale is falling amid record levels of violence. The use of new psychoactive substances is out of control - fuelling yet more violence. Mobile phones are flooding in, making the flow of drugs even more difficult to contain. So how does contraband make its way onto prison wings?

Former prisoners tell File on 4 that the bulk of smuggled goods come in with staff. Drones and visitors bring in small amounts, but the bigger consignments can only make it through with inside help. John Podmore, who's run jails and led the service's anti-corruption unit, says staff corruption is the inconvenient truth at the heart of the prison crisis.

"It is uncomfortable. They are few in number but they are large in their effect. One prison officer bringing in one coffee jar full of spice or cannabis can keep that jail going for a long time and make an awful lot of money."

Former prisoners tell the BBC's Home Affairs Correspondent, Danny Shaw, staff corruption is a serious problem but has become "the elephant in the room" that prison officials don't want to acknowledge. The ex-inmates say some staff are being corrupted while others turn a blind eye.

The Ministry of Justice has promised renewed efforts to combat corruption and professionalise the service. Thousands of frontline staff in London and south-east England will benefit from a pay boost, thanks to a new £12m package.

So will it stop the rot?

Reporter: Danny Shaw