Staff at Wormwood Scrubs have walked out over fears the prison has become "flooded with drugs, mobile phones and weapons".
Personnel have gathered outside the west London jail in protest, following a spate of recent incidents.
It comes weeks after an inspection report sparked claims of "Dickensian squalor" at the prison - one of the most famous in Britain.
It described the prison as rat-infested and overcrowded, with inmates spending most of their time holed up in squalid cells.
Mike Rolfe, of the Prison Officers' Association, said there had been a number of assaults on staff, while so-called "legal highs" were said to have been thrown over the walls.
Speaking from outside the prison, he said: "The jail is flooded with drugs, mobile phones and weapons.
"There are concerns for health and safety of staff, and they have decided to stay outside until they can seek assurances from management that their concerns will be addressed."
He said about 50 staff were outside the prison.
"Staff have said that if there's an emergency inside the prison, they will send teams in to deal with it," he added.
The action, which was not planned, was taken after an emergency branch meeting was held on Friday, Mr Rolfe said.
The Prison Service confirmed an incident is under way and steps are being taken to resolve it.
A spokesman added: "As a temporary measure, all prisoners will remain in cells and are being supervised. There is no danger to staff or the wider public."
Conditions at Wormwood Scrubs have come under fire from inspectors.
The recent scathing report revealed one in five prisoners told inspectors they felt unsafe, and the number of assaults on inmates and staff was double that seen in similar institutions.
Campaigners said at the time that the report revealed levels of "Dickensian squalor". Last year it was claimed that conditions had prompted one staff member to say: "I wouldn't keep a dog in there."
The episode will intensify scrutiny of the Government's attempts to reform the prison system.
Official figures have laid bare the rising tide of violence and self-harm behind bars.
Data covering England and Wales last week showed there were a total of 100 apparent self-inflicted deaths in the year to March - the highest level for more than a decade.
There were nearly 5,000 attacks on staff in 2015, a jump of more than a third compared with the previous year.
Earlier this year Prime Minister David Cameron set out wide-ranging proposals to overhaul the system, including the creation of six new "reform" prisons.
The shake-up will see the introduction of league tables to assess prisons, while efforts are being made to counter the availability of legal highs - which have been linked to the increasing levels of violence.
The Government has also announced plans to close old Victorian jails. Only HMP Holloway in north London has been confirmed as facing closure so far.
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