The government is conducting an inquiry into drug supply in prisons
An estimated £100m worth of drugs are being traded in prisons each year, an ex-prison service worker has said.
Former National Offender Management Service drug treatment head Huseyin Djemil said the Prison Service had no idea of the size of the drugs market.
He told BBC Radio 4's The Investigation the service needed to "get smarter" if it wanted to reduce drugs in prisons.
The government said the number of prisoners testing positive for drugs had fallen from 25% to 9% since 1997.
Mr Djemil estimated a worst case scenario of 20kg of drugs, mostly heroin, being smuggled into jails each week.
That amounts to more than 1,000kg, with a street value of about £100m, each year.
His analysis is backed up by figures from England's largest jail, Wandsworth, which estimated that about £1m worth of drugs were trafficked within the prison a year.
The government estimates that more than half the prison population are regular drug users - that would amount to more than 40,000 inmates.
Ministers are currently conducting an inquiry into ways of disrupting the supply of drugs in prisons. It is due to report at the end of May.
In one case of supply, last December a former prison guard and two inmates were sentenced for supplying drugs to a privately-run Warwickshire prison.
Former guard Gordon Hacker had admitted conspiring to smuggle cannabis into HMP Rye Hill between November 2004 and April 2005.
He had advised the inmates of the best times for drugs to be thrown over the prison wall.
In a separate case, in Peterborough last month Mark Berry was imprisoned after being convicted of supplying drugs to an inmate.
He had tried to pass methadone and cannabis to his ex-partner who was serving time in Peterborough prison.
The Investigation can be heard on BBC Radio 4 at 2000 BST on Thursday 10 April.