Ambulance unions have reacted with anger after the health secretary said they had “taken a conscious choice to inflict harm on patients”.
Steve Barclay said unions had refused to work with the government at a national level on how they would cover emergency calls during strike action.
Unison said it was “utterly shocked” by the comments, while the GMB union said they were “insulting”.
Paramedics are among those striking in England and Wales on Wednesday.
Control room staff and support workers who are members of the Unison, GMB and Unite unions are also involved.
Unions representing ambulance workers want pay increases to keep up with the rising cost of living. They have not set a specific figure but argue any offer needs to be high enough to prevent a recruitment crisis.
However, ministers are refusing to negotiate on pay, saying they have met the recommendations of the independent pay review body.
NHS bosses are warning patient safety cannot be guaranteed during the strike action, although unions say life-threatening callouts will still be responded to by an ambulance.
They also argue patients are already being put at risk due to waiting times and the pressure on the health service, made worse by staff shortages.
In an article for the Daily Telegraph, Mr Barclay accused ambulance unions of choosing to harm patients and making contingency planning more difficult.
He later told BBC Breakfast ambulance unions had chosen to strike at a time “when the system is already facing very significant pressure” from increased flu and Covid admissions.
Asked who would be responsible for any deaths during the industrial action, he said: “It is the trade unions who are taking this action at a point of maximum pressure for the NHS.”
But the unions involved in the dispute rejected Mr Barclay’s claims.
Christina McAnea, head of Unison, said in a tweet: “[The health secretary has] never specifically asked Unison for a national contingency agreement. In our meeting yesterday [Tuesday], he acknowledged that NHS staff – our local unions – have negotiated detailed, appropriate plans for their areas.”
She said the accusations were “a distraction from the government’s own failings and their refusal to constructively resolve this dispute”.
Rachel Harrison, national secretary of the GMB union, said ambulance workers felt “forced” to strike because the government had failed to listen to them.
“The sad reality is that patients are being harmed every single day, and that’s when we’re not on strike,” she added.
Sharon Graham, head of Unite, said there had been discussions for weeks about minimising the impact in emergency cases and “we don’t want patients to suffer in any way, shape or form”.
She accused Prime Minister Rishi Sunak of the worst “abdication of leadership” she had seen in 25 years, by refusing to negotiate on pay.