The UK’s biggest nursing union is preparing an escalation of its pay dispute with the government that will see members working in emergency departments, intensive care units and cancer care services being asked to join the next round of strikes.

The Royal College of Nursing (RCN) is also planning to announce the first continuous 48-hour strikes running through two days and two nights, rather than limiting walkouts to the 12 hours from 8am to 8pm, as they have done to date.

The union told NHS leaders on Friday of its intentions, and Downing Street is understood to have been informed, prompting alarm among senior officials and ministers.

The move by the RCN is aimed at finally breaking the deadlock with ministers over pay by taking action to a new level. The union says it will still make a priority of honouring its legal obligation not to endanger life.

To bolster support from members, it is also considering an increase in the level of strike benefit paid to its nurses, who lose a day’s wages from the NHS during action, from the current rate of £50.

While the RCN’s official pay demand has been for a 19% rise for this year, its general secretary, Pat Cullen, has made clear that she will call off action if ministers are prepared to discuss matching an offer of 7% from the Welsh government, which has led to the suspension of strikes there. So far, nurses in England have been given awards totalling only 4%, the level recommended by the NHS pay review body, and ministers have refused to offer more.

An RCN source said the plan to escalate action was designed to get NHS leaders to persuade ministers to meet the unions and talk about pay after months of refusing to do so.

Earlier this month, Cullen wrote to Rishi Sunak with a direct appeal for talks, as reported in the Observer, but the RCN says the prime minister has not replied and that there has been no contact with the Department of Health and Social Care.

The dates of the next strikes and the changes to the way the RCN will conduct them are expected to be announced within days, with the first round of new action happening within two weeks.

The union had agreed about 5,000 derogations, or exemptions, from strike action at a local level with NHS hospitals through joint committees of NHS and RCN staff. But it now says these will be stopped.

A union insider said: “NHS leaders are fearing this escalation and they must bring pressure to bear on government to get it stopped. They were expecting an escalation but had not prepared for the removal of the committees and derogation process that too many had manipulated at local level. We saw a minority of hospital management bullying nurses to break the last strike.”

This week, ballots of Unison ambulance workers who have yet to go on strike will close, meaning a potential escalation of action in ambulance services.

Last week figures from NHS England suggested about 137,000 appointments had been cancelled since the NHS strikes began last December. Unison says this will mean as many as 250,000 patients could face delays to appointments by late spring, should the government allow the dispute to drag on.

Unison’s head of health, Sara Gorton, said: “Dialogue can achieve great things. The prime minister should give it a go. Governments elsewhere in the UK have shown it’s good to talk to unions. That’s the only way disputes get resolved. Rishi Sunak says his door is open but without an invite to that mythical negotiating table, NHS staff have no choice but to go on striking.”

A Department of Health and Social Care source said: “It is disappointing that the RCN are escalating their strikes. Industrial action in the NHS is already having an impact on patients, with more than 80,000 appointments cancelled. These further walkouts, with less strike-day cover being promised, will worsen that impact and put more patients at risk.

“The health and social care secretary has been clear he wants to continue discussing with unions what is fair and affordable as part of the 2023/24 pay process, including concerns around pay, conditions and workload to find ways to make the NHS a better place to work for everyone.

“Our priority is keeping patients safe. The NHS has tried and tested plans in place to minimise disruption and ensure emergency services continue to operate.”