Prison officers staged walkouts over past five months
Prison officers have withdrawn labour or staged unofficial walkouts at five prisons in England over the past five months, the BBC has learned.
The Prison Officers Association, whose members are banned from going on strike, said the action was the result of staff concerns over their safety.
Last year there were nearly 5,000 assaults on prison staff - the highest number for at least a decade.
The Ministry of Justice has announced almost £13m in funds to improve safety.
This will be available in 69 prisons, including four of those were staff have taken action.
Although two of the staff walkouts at Wetherby young offenders institution in West Yorkshire and Wormwood Scrubs in west London were known about, details of the other three have only just emerged.
In May, four members of staff refused to carry out their duties at the Mount prison in Hertfordshire, while 40 prison officers at Holme House jail, in Stockton on Teesside, withdrew in protest about changes to the regime.
By Danny Shaw, BBC home affairs correspondent
Largely unnoticed and seldom praised, they do the work many of us would not want to do, dealing with some of the most dangerous and damaged people in society.
So when prison officers in five separate jails down tools it's important we understand why.
There is a rising tide of mental illness, aggression and violence in prisons - borne out by official figures published every quarter - that staff are struggling to cope with.
The extra money for safety improvements released by Justice Secretary Michael Gove will be welcomed.
But it's likely to take more than that to keep the problems in check - and ensure that Mr Gove's ambitious plans for "reform" prisons remain on track.
The latest unofficial action occurred last month, when officers on two wings at Swinfen Hall in Staffordshire left prisoners locked in their cells because of safety concerns.
Prison Officers Association chairman Mike Rolfe said the walkouts were because members feared a "significant and imminent threat to their health and safety".
He said there was a shortage of staff, which meant most time was taken up with formal duties such as locking and unlocking prisoners and searching for items, leaving little time for informal interactions between staff and prisoners.
"There's a massive divide between prisoners and officers," he added.
A Ministry of Justice spokesman said it was "wholly inaccurate" to say there have been "significant disputes" at Holme House, Swinfen Hall and the Mount prisons.
He added: "The safety and welfare of our staff is a key priority and we will always ensure prisons have enough staff to run safely and securely.
"Since January last year we have recruited 2,830 prison officers nationally, a net increase of 530 officers.
"We have also secured £1.3bn to modernise the prison estate and are investing an additional £10m to help respond to safety issues."