Monitors at HMP Pentonville found old windows had not been replaced, vermin was rife and prisoners went weeks without exercise in the fresh air.
The assessment comes days after a scathing critique of another large Victorian prison, HMP Birmingham.
Pentonville’s Monitoring Board said the north London prison, which hit the headlines two years ago when two prisoners staged an audacious escape, remains “porous”.
Windows flagged up in 2016 are still insecure and compromise the safety of staff and inmates, according to the MB’s annual report for 2017-18.
It found that, despite a recommendation, not a single external window grille had been replaced.
There had been an increase in gang-related incidents during gatherings for prayer. On one occasion, a fight erupted and ministers had to run for cover.
The board said Pentonville has many energetic and committed staff but there were too few officers for most of the year.
Wings were shut down for three or four half-days a week, activities and association time were restricted and some prisoners went weeks without exercise in the fresh air.
The report said: “Pentonville is in the ‘Top 10′ of prisons most in need of investment.
“Twelve hundred men live in a building certified to hold nine hundred. Vermin is rife.
“Persistent overcrowding and the crumbling physical environment are incompatible with maintaining prisoners’ humanity and dignity.”
Opened in 1842, Pentonvillle is one of the country’s busiest prisons, with about 33,000 movements a year through its reception.
At the end of last month, the state-run jail was holding 1,215 men.
Safety and conditions behind bars in England and Wales have been under the spotlight since chief inspector of prisons Peter Clarke warned HMP Birmingham had fallen into a state of crisis.
In a report published on Monday, Mr Clarke detailed “appalling” squalor and violence at the privately-managed prison, which the Government has now taken over.
A Prison Service spokesman said: “We are investing £16m across the estate to bring prisons back up to acceptable standards, and work is underway to fix Pentonville’s old windows and grilles with around 30 per cent already replaced.
“The prison is seeing a reduction in drug use thanks to new netting, as well as regular sniffer dog and staff-led searches.
“In addition, 35 new prison officers have been recruited and we are working with charities to better identify and rehabilitate known gang members at Pentonville.
“The problems in our prisons will not be fixed overnight. But reducing crowding is a central aim of our modernisation plans – precisely why we have committed to delivering up to 10,000 new prison places across the country.”
Mr Leech said: “I find it astonishing that the Ministry of Justice has to chutzpah to operate prisons like this, and still look the public in the eye.
“If the 18th century prison reformer John Howard could walk around Pentonville today he would recognise many of the things he condemned 176 years ago – Charles Dickens wrote books about prisons like this.
“As Oscar Wilde once remarked: ‘If this is how Her Majesty treats her prisoners, then she doesn’t deserve to have any.”