Tens of thousands of nurses are going on strike today for their first mass walkout in a century across England, Wales and Northern Ireland.
To read the latest on the nurses' strike, follow latest here:
Thousands of NHS appointments and operations cancelled as nurses' strikes begin
The action, a bid to secure above-inflation pay rises, is going ahead after talks to avert it ended in a deadlock.
Picket lines are being set up at dozens of hospitals and thousands of NHS appointments and operations have been cancelled, with the health service running a bank holiday-style service in many areas.
The Royal College of Nursing (RCN) has said it will still staff chemotherapy, emergency cancer services, dialysis, critical care units, neonatal and paediatric intensive care.
Some areas of mental health and learning disability and autism services are also exempt from the strike, while trusts have been told they can request staffing for specific clinical needs.
When it comes to adult A&E and urgent care, nurses will work Christmas Day-style rotas.
Saffron Cordery, interim chief executive of NHS Providers, said agency NHS trusts were "pulling out all the stops" to lessen the impact on patients.
She said: "But it's inevitable that some operations or appointments will have to be rescheduled, and trusts are pulling out all the stops to minimise disruption.
"The cold snap has ramped up demand that was already at or close to record levels, but on strike day NHS trusts will do everything they can to ensure that essential services are properly staffed and patient safety, always the number one priority, is safeguarded."
RCN chief executive Pat Cullen has accused Health Secretary Steve Barclay of "belligerence" after he refused to discuss the issue of pay - because the government has already accepted recommendations made by the NHS Pay Review Body (PRB) to give below inflation pay rises of around 4%.
This would have seen them get a pay rise of around £1,400.
The RCN has been calling for a pay rise of 19% - 5% above inflation - though it has indicated it would accept a lower offer.
When it submitted the 5% figure to the independent pay review body in March, inflation was running at 7.5%.
But inflation has since soared, with RPI standing at 14.2% in September.
19% is unrealistic
Health minister Maria Caulfield told Sky News Health Secretary Steve Barclay did discuss pay with the RCN during a meeting on Monday, despite ministers over the weekend insisting it was not for the government to negotiate pay but for the independent pay review bodies.
But she added: "We can't go to 19%, that's unrealistic.
"We could have ignored the pay review bodies recommendations and gone for a much lower pay rise but we did it higher.
"We could go higher but we have got to find that money from somewhere - this isn't government money, this is taxpayers' money.
"The three options open to us if we were to have to fund a higher pay rise is either to borrow the money, tax people more, or take that money from frontline staff."
However, Ms Cullen said Mr Barclay did everything but talk about pay during the meeting and he made her wait half an hour.
'A tragic first'
Meanwhile, in Scotland, RCN members are being consulted on a revised pay offer from the Scottish government.
Ms Cullen said: "Nurses are not relishing this, we are acting with a very heavy heart.
"It has been a difficult decision taken by hundreds of thousands who begin to remove their labour in a bid to be heard, recognised and valued.
"It is a tragic first for nursing, the RCN and the NHS.
"Nursing staff on picket lines is a sign of failure on the part of governments.
"My plea to patients is to know that this strike is for you too - it's about waiting lists, treatments that are cancelled year round and the very future of the NHS."
The RCN has also raised the issue of huge staff vacancies in the NHS, with 47,000 nurse roles empty in England alone. And it has warned strike action may need to continue into January if the government does not re-negotiate on pay.
The health secretary said nurses were "incredibly dedicated to their job" and that it was "deeply regrettable some union members are going ahead with strike action".
Mr Barclay added: "My number one priority is to keep patients safe - I've been working across government and with medics outside the public sector to ensure safe staffing levels - but I do remain concerned about the risk that strikes pose to patients.
"Nevertheless, the NHS is open and patients should continue to seek urgent medical care - and attend appointments - unless they've been contacted by the NHS."
He said paying nurses more "would mean taking money away from frontline services at a time when we are tackling record waiting lists as a result of the pandemic."
But pressure is mounting on the government to find a compromise on pay, with former Conservative Party chairman Sir Jake Berry saying it "is going to have to improve its offer".
"We need to find a way as a government, and the union does too, to get to that centre point, that point of agreement straight away," he told Talk TV.
During the strike, nurses will man picket lines at major NHS hospitals, including Guy's and St Thomas NHS Foundation Trust in London, Addenbrookes Hospital in Cambridge and University Hospitals Birmingham NHS Foundation Trust.
Several trusts have already given details of cancelled outpatient appointments and planned treatments.
The Western Trust in Northern Ireland said it had "regrettably taken the decision to cancel some non-emergency services", with 587 outpatient appointments postponed across Altnagelvin Hospital, Omagh Primary Care and Treatment Centre and South West Acute Hospital.
The trust said there would also be reduced staffing in community nursing services including rapid response nursing, district nursing, community respiratory nursing and continence services.
Who is striking this winter and why?
In Wales, the Welsh government said non-urgent or routine appointments are likely to be postponed.
On Wednesday, the head of NHS Employers said "real concerns" remain about the level of cover nurses will provide for cancer patients during the strike.
In a letter to NHS leaders, Danny Mortimer said some aspects of talks with the RCN had been disappointing and warned that "unless the government indicates a willingness to negotiate on pay-related matters, further strike dates will be announced by the RCN for January 2023 and beyond".
A second RCN nurse strike is set for 20 December, while thousands of ambulance workers will go on strike on 21 December.
The RCN has urged agency workers not to cover for striking staff.
Members of the Rail, Maritime and Transport union (RMT) began the first of two 48-hour strikes at Network Rail - and 14 train companies - on Tuesday, which will last until Friday.
There is also industrial action planned in a whole number of UK spheres, including paramedics, postal workers, Border Force agents, firefighters, driving instructors, bus operators, airport baggage handlers and even coffin makers.