Angela Levin, who worked at HMP Wormwood Scrubs for a decade, has laid bare the shocking state of the capital's prisons.
She said last October's cuts, which saw 20 per cent slashed off the prison budget, have left the system close to breaking point.
In Wormwood Scrubs, where she headed the Independent Monitoring Board for ten years, she said that over a fifth of staff had lost their jobs, leaving as little as five officers for 300 prisoners in some wings.
Ms Levin, who has written a book detailing her experiences at the west London prison, also said that at any time around a quarter of inmates were detainees waiting to be removed from the country.
She said: "London prisons are clogged up with detainees waiting to be deported, there are hundreds of them.
"They languish in the system waiting for their embassies to give them permission to be deported, and waiting for the paperwork to go through.
"They could be held in prisons in the country where it's less crowded. They haven't got family who want to visit them in London, and that would reduce the numbers being held in overstretched prisons."
Ms Levin also said that drugs are endemic in prisons, and described being offered heroin by an inmate who knew that she was an official.
She said that she and been trying to get a prison transfer for the man, who had been angry in their last encounter because he wanted to be moved north to be nearer his family ...... m o r e
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