Ambulance staff across most of England and Wales will go on strike on 21 December in a dispute over pay.
The coordinated walkout by the three main ambulance unions - Unison, GMB and Unite - will affect non-life threatening calls only.
But it could mean people who have had trips and falls not being responded to.
Members of GMB, which represents nearly a third of the 50,000-strong workforce, will then follow that up with another walkout on 28 December.
It comes as Royal College of Nursing members are also preparing to go on strike on 15 and 20 December in parts of England, Wales and Northern Ireland.
The walkouts will involve paramedics as well as control room staff and support workers, with the military on standby to help out.
The only service which will be completely unaffected, however, is the East of England.
Under trade union rules, life-preserving care has to be provided so the two highest category calls - covering everything from heart attacks and strokes to major trauma - will still be responded to.
But Matthew Taylor, of the NHS Confederation, which represents health managers, said he was worried the action would "undoubtedly" affect patient care and how quickly ambulance services could respond and may even deter people from seeking help.
"The prospect of industrial action over Christmas is very concerning," he added.
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Source: BBC News, 6 December 2022