Thousands of ambulance workers in England will strike on 10 February, Unison has announced.
The union said workers across five services will walk out as the long-running dispute over pay and staffing continues.
Ambulance workers in London, Yorkshire, the South West, North East and North West will be involved in the industrial action.
Strikes will now be happening across the NHS every day next week apart from Wednesday.
‘Toxic cocktail’ behind dire economy forecast – politics latest
Downing Street said the latest strike action by ambulance workers is “deeply concerning”.
“We are putting in place significant mitigations that have previously helped reduce some of the impact from these strikes,” the PM’s official spokesperson said.
“But first and foremost we would ask the unions to reconsider that approach and continue discussions.”
Announcing the latest industrial action, Unison urged ministers to stop “pretending the strikes will simply go away” and improve ambulance workers’ pay.
The union also warned that unless the government has a “major rethink” over NHS pay, and gets involved in “actual talks” with unions, it will announce strike dates running into March.
It added that by then, the dispute is likely to affect double the number of trusts and extend to the whole of the ambulance service in England.
“The government must stop playing games. Rishi Sunak wants the public to believe ministers are doing all they can to resolve the dispute. They’re not,” the union’s head of health Sara Gorton said.
“There are no pay talks, and the prime minister must stop trying to hoodwink the public. It’s time for some honesty. Ministers are doing precisely nothing to end the dispute.
“The government’s tactics seem to be to dig in, wait months for the pay review body report and hope the dispute goes away. It won’t. And in the meantime, staff will carry on quitting, and patients being let down.
“There can be no health service without the staff to run it. Ministers must open proper talks to end the dispute and put in place the urgent retention plan needed to boost pay and staffing across the NHS.”
The latest strike announcement comes as ministers are braced for the biggest day of industrial action in over a decade on Wednesday when teachers, university lecturers, train drivers, civil servants, bus drivers and security guards stop work on the same day.
The PM’s spokesperson conceded that Wednesday’s mass strike action will be “very difficult”.
Members of the Fire Brigades Union (FBU) voted overwhelmingly in favour of action in a ballot result announced on Monday.
The FBU has said it is giving the government and employers 10 days to make an improved offer before deciding its next move.
If they go ahead, the strikes will be the first nationwide fire strikes over pay since 2003.
Read more:
Public support for strikes rising, Sky News poll shows
PM says he ‘acted decisively’ in sacking Zahawi
Watchdog chief removes himself from BBC chairman probe
While the National Education Union (NEU) accused the government of having “squandered” the chance to avert a walk-out in schools in England and Wales after talks with Education Secretary Gillian Keegan broke up without agreement.
New Sky News polling shows support for trade unions is rising even though strike action is bringing public services to a standstill.
Meanwhile, NHS consultants in England are also gearing up for possible strike action.
The British Medical Association (BMA) – the country’s biggest doctors’ union – is to hold an indicative ballot of its consultant members in February in a dispute over pay and pensions.
The BMA has said that while the consultative poll – which opens on 10 February and closes on 27 February – was not a formal ballot, it represents a significant escalation towards one.
Around 45,000 junior doctors who are members of the union have also been balloted over strike action – with the result due at the end of February.
Speaking on Monday, Saffron Cordery, interim chief executive of NHS Providers, said the threat of more strikes was “alarming” in a service already struggling to cope with the effects of the most widespread industrial action in its history.
A new anti-strike law – which would see minimum service levels set for fire, ambulance and rail services for when the sectors decide to take industrial action – cleared its final Commons hurdle on Monday evening.