The biggest health union is beginning to ballot its members in England, Wales and Northern Ireland on strike action.
Unison is asking 350,000 NHS staff, including porters, nurses, paramedics and cleaners, to vote in favour of walking out in a dispute over pay.
But a ballot of its 50,000 members in Scotland, which was already under way, has been suspended after a new offer.
The government in England has urged staff to consider the impact of a walkout on patients.
It pointed out it had given staff a rise in line with what had been independently recommended by the official NHS Pay Review Body.