Health workers in Northern Ireland and England, including midwives, nurses, radiographers, cleaners and psychiatric staff, staged a four-hour walkout today over pay.
Members of 11 unions began their four-hour walk out at 7am in England and 8am in Northern Ireland.
They are protesting at the British government's decision not to accept a recommended 1% pay rise for all National Health Service employees.
Unison leader Dave Prentis said the strike, the second in a month, should "sound alarm bells ringing" in Westminster because of the anger of such dedicated workers.
Unions have struck deals with the administrations in Scotland and Wales, increasing their anger at Health Secretary Jeremy Hunt, who they accuse of refusing to discuss the long-running row.
The minister has erected a "Berlin wall of intransigence", the unions said.
Mr Prentis earlier said: "The fact almost all health unions are taking part in the industrial action should ring alarm bells in Whitehall.
"We are only asking for decent pay for the hard-working people the government say they care so much about.
"There will be plenty of opportunities for Jeremy Hunt to go in trusts around the country to meet with staff who will be working to rule, taking away the goodwill the NHS relies on so much.
"NHS workers in Scotland and Wales will all be getting a 1% pay rise and the Living Wage. So why is the Secretary of State so determined to penalise workers in England?"
By the end of the next financial year, health workers will have had their pay capped for six years, said the TUC.
General Secretary Frances O'Grady joined Mr Prentis on a picket line at the London Ambulance Service headquarters in Waterloo as a show of support for health workers taking part in the pay protest.
She said: "Health workers care passionately about their patients and the quality of service they provide, and so are always reluctant to take action.
"But the Government's refusal to accept the recommendations of NHS independent pay review body - even though it only called for an affordable, below-inflation pay rise - leaves health workers feeling that they have no other option.
"It's not too late for the Government to change course and award health workers the pay rise the public knows they deserve."
Cathy Warwick, chief executive of the Royal College of Midwives, said: "I was overwhelmed by the response of our members to the previous industrial action and I urge them to respond equally next week.
"I am also very heartened to see that public support for a 1% award for NHS staff has remained high since that industrial action, so we know the public are behind us.
"This is not about our members demanding huge banker-sized bonuses or asking for the similarly large bonuses and pay increases given to many senior managers in the NHS.
"It is about our members having to fight just to get the very modest 1% pay award recommended by the NHS Pay Review Body.
"It is also an award which still lags way behind the rising cost of living and will see our members earning the same in 2016 as they did in 2013.
"As before, in every area our local representatives have worked with hospitals to ensure safe services will be available to women in need of urgent care, such as those in labour.
"Our dispute is not with the women for whom midwives care, it is with employers telling midwives they are not worth a 1% pay rise."