A mass walkout by prison staff led to the adjournment of hundreds of criminal cases at an estimated cost of more than £100,000, provisional figures show.
More than 100 prison officers at Birmingham Prison joined colleagues across the country in a protest over concerns about violence and safety in jails.
The Prison Officers Association said there's been an unprecedented decline in safety and security in prisons nationally with assaults against prison officers up 108% since 2010.
The walkout meant 285 Crown Court cases were postponed after defendants could not be brought from custody on September 14 because of protests over violence in jails.
Members of the POA, the trade union for prison staff, demonstrated to oppose working in "the most hostile environment workplace in Europe", chairman Mark Fairhurst said.
Mr Fairhurst said the figures underlined "how essential our job is to the public" and called for prison officers to be treated like other emergency services.
He said: "We're the forgotten service who work behind a big grey wall. Nobody knows what we do unless there's adverse publicity ... that's when it gets out what life for us is really like.
"But we are an essential public service which has just been highlighted by those statistics alone."
Responding to the findings, a Ministry of Justice (MoJ) spokeswoman said:
Thousands of prison staff took part in the walkout, which was called off after "meaningful engagement" with prisons minister Rory Stewart, the union said.
The figures provided by the MoJ were collated in response to a Freedom of Information request and are not classified as official statistics.
Mr Fairhurst welcomed some new measures, saying that, in the coming months, prison staff would be equipped with the PAVA incapacitant, which is similar to pepper spray, as well as rigid police-style handcuffs and new personal protection equipment.
But he said the service was struggling with high turnover of inexperienced staff owing to relatively low pay, the daily threat of violence and a retirement age of 68 as opposed to 60.
Mr Fairhurst said he wanted to see airport-style security and search teams at every jail in England and Wales, and that dog units could help intercept drugs thrown over prison walls.
Standards across the prison estate have come under intense scrutiny in recent years amid a slew of highly critical reports and a deterioration in safety measures.
Chief Inspector of Prisons Peter Clarke warned staff and inmates have become "inured" to conditions unacceptable in 21st century Britain, in his annual report for 2017/18.
He highlighted how thousands of inmates are living in squalid and overcrowded cells, locked up for nearly 24 hours a day.
Official figures published in October revealed that assault and self-harm incidents were continuing to rise, both reaching new record highs.
The maximum sentence for anyone who assaults prison officers was doubled in November, from six months to one year.
The MoJ spokeswoman added: "We take prison officers' welfare seriously, which is why are spending an extra #70 million on improvements to safety, security and decency, providing officers with pepper spray, and have recruited over 4,000 new officers in the last two years."