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.
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 impact of the ambulance walkout will be patchy as Unison, which is the largest union in the sector, only has a strike mandate for half of England’s 10 regional services.
GMB has also announced its members will take strike action on 28 December.
The walkouts will involve paramedics as well as control room staff and support workers, with the military on standby to help out.
The start times and lengths of the walkouts vary between unions and services, but all will last between 12 hours and 24 hours.
The only service which will be completely unaffected, however, is the East of England.
Unison head of health Sara Gorton said: “The government will only have itself to blame if there are strikes in the NHS before Christmas.
“Ambulance staff and their health colleagues don’t want to inconvenience anyone. But ministers are refusing to do the one thing that could prevent disruption – that’s start genuine talks about pay.
“Wages are too low to stop health workers quitting the NHS. As more and more hand in their notice, there are fewer staff left to care for patients. The public knows that’s the reason behind lengthy waits.”
All the unions have asked for above-inflation pay rises.
But the governments in England and Wales have given NHS staff an average rise of 4.75%, with staff guaranteed a minimum of £1,400.
England Health Secretary Steve Barclay said paying what was being asked for was “unaffordable” and he has pointed out the award met the requirements of the recommendations of the independent NHS Pay Review Body.
About two thirds of ambulance staff are members of one of the three unions.
The last time ambulance staff were involved in strike action was in 2014.