Nurses from A&E, intensive care and cancer wards could strike in England, as a major nursing union considers escalating a dispute over pay.
The Royal College of Nursing (RCN) wants higher wages as the cost of living bites, but ministers argue that pay is set by an independent body.
The union is considering a continuous 48-hour strike, with walkouts through the night.
However, all staff would continue to meet minimum legal levels of care.
The union also said it may not make local agreements to help NHS managers during the strike periods, as it has done before.
In previous action, there were about 5,000 cases where the strike was not fully implemented which were decided through joint committees of NHS and RCN staff.
A Royal College of Nursing source said NHS leaders are “fearing this escalation” and “must bring pressure to bear on government to get it stopped”.
The union would have to give two weeks’ notice for any action.
The RCN is frustrated that the government has refused to discuss increasing the 4% pay offer already made for the current 2022/23 year.
The Department for Health has indicated there is no change in its position for nurses in England, including on pay for the current year – one of the main contentions in the dispute.
It said Health Secretary Steve Barclay would discuss pay for the coming year, and urged unions to call off their strikes.
“Our priority is keeping patients safe,” the department added, saying the NHS has “tried-and-tested plans” to minimise disruption.
Earlier this month, the RCN’s general secretary Pat Cullen wrote to Prime Minister Rishi Sunak asking for talks. The union said it has had no reply.
On Saturday night, Downing Street said it would not add another level of negotiation into the process, and the health secretary would respond to the letter in due course.