Ambulance workers on the picket line outside Croydon Street Ambulance Station in Bristol, England, Monday, Jan. 23, 2023. AP
Ambulance workers began strike action on December 21 last year and further dates are planned for February.
Nurses have also launched unprecedented strike action, reflecting widespread discontent in the state-funded National Health Service whose staff are struggling with soaring living costs.
Monday's industrial action comes ahead of mass simultaneous strikes planned for February 6 by unions representing both nurses and ambulance staff.
Monday's strike involves members of three unions -- Unison, Unite and GMB -- in England and Wales.
Unison, the UK's largest trade union, said up to 15,000 ambulance staff in England would be striking along with 5,000 members at hospitals in Liverpool in northwest England.
Unite said more than 2,600 of its ambulance worker members were striking in England and Wales, following a three-day strike last week.
"We're trying to get the government to negotiate," Unite general secretary Sharon Graham told BBC radio, accusing the government of inaction.
"It's been five weeks since Unite ambulance workers were out and in that time there has been literally no meeting about the substantive issue of pay.
"There are absolutely no pay talks going ahead, it's wrong to suggest that they are," she added, even as Health Secretary Steve Barclay has said discussions were ongoing.
Prime Minister Rishi Sunak is "missing in action" and "needs to get more involved," she added.
The GMB, whose members at one ambulance service in central England were striking Monday, addressed Barclay in a tweet saying: "We need to talk pay. We're ready when you are."
The London Ambulance Service advised people to only call the 999 emergency number in "life-threatening" situations and warned the public they "may be asked to organise transport to hospital".