Members of the Royal College of Nursing (RCN) are due to walk out for 48 hours from 8pm on Sunday night until 8pm on Tuesday 2 May after rejecting the latest pay offer from the government.

But Steve Barclay, the health secretary, has written to RCN boss Pat Cullen, saying the union's current six-month strike mandate runs out at midnight on 1 May.

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After talks to solve the issue last week failed, Mr Barclay said: "I therefore regretfully provided notice of my intent to pursue legal action with a view to protecting patients, NHS workers and RCN members whilst continuing to seek a way to resolve this through official channels."

Ms Cullen said the RCN had told the government such action was "wrong and indefensible" but "the threat sadly became a reality".

In an email to members, she added: "The only way to deal with bullies is to stand up to them - including in court.

"It's so wrong for the government to use taxpayers' money to drag our profession through the courts.

"We're determined to show that the nursing profession is strong and determined and defend our members' right to strike."

In a statement released after the news broke, Mr Barclay said he had been "left with no choice but to proceed with legal action".

He added: "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."

Ms Cullen confirmed members would not be asked to walk out if the court ruled against them, saying: "If the government succeeds in silencing members like you and convinces the court to stop part of our strike, then we'll have no choice but to cut it short."

She added: "Our strike action has always been safe and legal. We would never ask our members to do anything unsafe or against your professional code."