Prison officers in England and Wales have voted to take industrial action in a row over pay and bargaining rights.
The union says prison officers are "incensed"
With only a handful of votes left to be counted, nearly 15,000 members have backed action and 2,400 voted against, the Prison Officers' Association said.
The union said the dispute was over the independence of the pay review body and claims its members have seen a pay cut.
The Prison Service said it was a matter of grave concern that the POA might act in breach of a legally-binding accord.
The Northern Ireland wing of the union said it also planned to join industrial action.
However, the Prison Service said it and the POA had signed the industrial relations procedural arrangement (JIRPA), which included an agreement by the POA not to take industrial action.
The union needed to give 12 months' notice if it wanted to withdraw from this agreement, said the service.
"If necessary the Prison Service will seek legal remedy," it said.
"Clearly the Prison Service is disappointed, although we have not been officially informed of the result of the POA ballot," it said.
POA National Chairman Colin Moses said: "What we now see is clear interference in the pay review body and a 1.6% pay award imposed in April this year. Our members have taken a pay cut.
"Prison officers are incensed and their morale is at an all-time low."
Mr Moses also rejected the idea that it would be unlawful for prison officers to take action because it would contravene the agreement between the POA and the Prison Service.
"Pay stands outside of that agreement. They introduced what should have been an independent pay review body. We would argue that the government and the Prison Service have breached that agreement," he said.
He said the union was now asking "first and foremost" for talks with Prisons Minister Gerry Sutcliffe
The 29,000-strong union balloted members in 132 public sector prisons across England and Wales.
But Finlay Spratt, Chairman of the POA in Northern Ireland, said its members had supported industrial action in a ballot two years ago over pay and conditions and the issue was not resolved.
"If colleagues in England and Wales take industrial action, my wish is that we would be part of that," he said.
He said prison officers had seen the value of their pay fall around 5% over the last three years.
"The least we expect is for pay to keep in line with inflation," he said. "Prison officers do a very dangerous job and it is time society stood up and said this isn't good enough."