The NHS has launched a legal challenge that could end in the high court to block the second day of an upcoming strike by tens of thousands of nurses. Officials at NHS Employers wrote to the Royal College of Nursing (RCN) on Wednesday saying the union’s plans for a two-day strike were unlawful. In response, it is understood the RCN has said it will “forcibly resist” employers’ attempts to seek a high court injunction designed to block the strike, which they insist is lawful. The threat raises the possibility of a high court clash between NHS lawyers backed by the government and those of the nursing union. It also highlights the increasingly bitter relationship between the government and those representing workers on the frontline of the health service. The country’s largest nursing union secured support for strikes in a vote of its members on 2 November. The union has six months to take the industrial action and the NHS claims the second day of the planned strike falls outside the mandate. It is understood lawyers for the RCN responded to the claims in a letter to NHS Employers on Wednesday. The planned strike action is due to take place from the start of the night shift at 8pm on Sunday 30 April until 8pm on Tuesday 2 May. The RCN is relying on a 1995 mining dispute to argue that the strike action can lawfully extend to midnight on 2 May. According to a letter seen by the Guardian, it has said it is willing to go to court to fight its case. It wrote: “It will be forcefully resisted by our leading counsel retained on this matter and we would also seek to recover our costs if any such application is unsuccessful, which I believe it would be.” After RCN members voted by 54% to 46% to reject the government’s offer of a 5% pay rise this year and a cash payment for last year, the union announced the 48-hour strike starting on 30 April. RCN members have since rejected the offer of a one-off payment worth between £1,700 and £3,000 for 2022-23 and 5% for 2023-24, with 54% voting against it on Friday on a 61% turnout. The RCN has recently announced it will seek a second legal mandate to continue its strike action from June to December, raising the stakes for the government. Despite Pat Cullen, the RCN general secretary, requesting a meeting with the health secretary, Steve Barclay, this week, none has taken place. The government is facing some opposition from backbenchers over its stubborn stance. On Wednesday night, the former Tory party chair Jake Berry suggested the health department should reopen negotiations to avoid more damaging action. Daniel Mortimer, the chief executive of NHS Employers, said: “The RCN ballot for industrial action ended at midday on 2 November 2022 and allows the union six months to undertake any action approved by that ballot. “NHS Employers has written to the RCN stating our view – on behalf of trusts in England, and with clear legal advice – that the RCN’s mandate for industrial action ends at midnight on Monday 1 May. We have therefore asked the RCN to amend its guidance to its members regarding any action planned for Tuesday 2 May 2023. We are in ongoing exchanges with the RCN on this matter.”