“Management and Government cannot continue to turn a blind eye to the drug culture which is making our prisons unsafe.”

A report by Peter Clarke, the chief inspector of prisons, found last month that Wormwood Scrubs suffered from squalid conditions and the number of assaults on inmates and staff was double the level of comparable institutions.

Some prisoners told inspectors they were too frightened to leave their cells.

Mike Rolfe, a POA member, said about 50 staff had gathered outside the jail, which was built between 1875 and 1891.

"The jail is flooded with drugs, mobile phones and weapons,” said Mr Rolfe.

"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."

The action was taken after an emergency branch meeting was held on Friday, Mr Rolfe said.

A Prison Service spokesman said: "As a temporary measure, all prisoners will remain in cells and are being supervised. There is no danger to staff or the wider public."

Data covering England and Wales, published last week, showed there were nearly 5,000 attacks on staff in 2015, a jump of more than a third compared with the previous year.