Thousands of NHS workers across England and parts of Northern Ireland have walked out for four hours in a row over pay.
The hospital staff, including midwives, nurses and paramedics, are protesting against the government's decision not to award all NHS workers a 1% pay rise, which was recommended by the NHS pay review body earlier in the year.
The workers, who stressed that "life and limb" services will not be affected, are walking out between 7am and 11am on 13 October.
In addition, the trade union members will not be doing paid overtime and will be taking breaks as part of the industrial action.
"This government's treatment of NHS workers has angered them and this anger has now turned into action," said Dave Prentis, the general secretary of Unison.
"Refusing to pay them even a paltry 1% shows what the government really thinks about its health workers.
"Inflation has continued to rise since 2011 and the value of NHS pay has fallen by around 12%."
But the government has argued that increasing pay for all NHS workers would mean jobs would have to be axed.
Jeremy Hunt, the health secretary, told BBC Radio 4's Today Programme that a pay hike would have cost 4,000 jobs in the NHS this year.
"We are disappointed that Unison is planning industrial action and has rejected our proposals to give NHS staff at least one per cent pay this year and at least a further 1% next year," a spokesperson for the Department of Health said.
"NHS staff are our greatest asset and we know they are working extremely hard.
"This is why despite tough financial times, we've protected the NHS budget and now have 13,500 more clinical staff than in 2010.
"We want to protect these increases and cannot afford incremental pay increases - which disproportionately reward the highest earners - on top of a general pay rise without risking frontline NHS jobs.
"We remain keen to meet with the unions to discuss how we can work together to make the NHS pay system fairer and more affordable."
St Thomas' Hospital Strike
IBTimes UK visited a picket line outside of St Thomas' Hospital in London, a stone's throw from the Houses of Parliament.
Declan Simmington, an antenatal education midwife at St Thomas', said he was not convinced by Hunt's warning.
"The independent NHS pay review body gave the 1% pay rise recommendation and one of the things the body looks at is costing – that it's going to be affordable for the NHS to provide it."
"If you have a demotivated workforce, then they don't work as efficiently. So paying the 1% may actually save the NHS some money."
Fergal O'Reilly, a steward for Unison and a NHS admin manager, said the government's decision not to award all NHS staff the 1% rise has been "very demoralising".
"The job that our members do in the NHS is very important one way or another, and morale is naturally affected by that," O'Reilly told IBTimes UK.
"We are doing this because we care about the NHS, we are doing it for the right reasons."