A massive walkout by nurses and ambulance workers on Monday weakened Britain's state-run NHS with its largest ever strike.
Physiotherapists are supposed to walk out on Thursday as well, making the week the most disrupted in NHS history.
Health workers are demanding a pay rise that reflects the worst inflation in Britain in four decades.
Around 500,000 workers, many from the public sector, have been staging strikes since last summer, adding to pressure on Prime Minister Rishi Sunak.
Both sides have failed to reach an agreement, despite weeks of negotiations, despite the RCN calling for a 5% pay raise above inflation.
Not all ambulance workers will strike at once and emergency calls will be attended to.