The U.K. government on Monday said it was applying for a court order to stop a strike by nurses in the state-run National Health Service from going ahead.
Health Secretary Steve Barclay said he was acting on a request from NHS Employers, which represents hospital groups in England and Wales, to declare the planned walk-out on May 2 "unlawful."
Nurses are due to start a new strike on the evening of April 30 and end two days later, on May 2. The government is arguing that the nurses’ union mandate for a walkout covers only the first day of the planned strike, and thus the second day would be illegal.
"Despite attempts by my officials to resolve the situation over the weekend, I have been left with no choice but to proceed with legal action," Barclay said.
"I firmly support the right to take industrial action within the law — but the government cannot stand by and let a plainly unlawful strike action go ahead nor ignore the request of NHS Employers.
"We must also protect nurses by ensuring they are not asked to take part in an unlawful strike," he said.
Members of the Royal College of Nursing (RCN) earlier this month voted to reject a 5% government pay offer to end their industrial action.
Union bosses then announced an escalation of walkouts to include staff in emergency departments, intensive and cancer care units for the first time.
But that has led to fears about the provision of critical care for patients, and the worsening of a growing backlog in appointments and procedures.
The government is relying on legislation that stipulates unions have six months from the initial vote for strike action to walk out.
It maintains the RCN's lawful ballot mandate ends at 11:59 pm on May 1.
The RCN has accused NHS Employers of "seeking to discredit" the action on May 2 and will "strongly resist any application for injunctive relief from any or all NHS employers."