UK health secretary Steve Barclay on Wednesday doubled down on the government’s decision not to negotiate on pay with striking NHS staff, as he accused ambulance unions of jeopardising patient safety.

The government is braced for severe disruption within the NHS across England and Wales, following the decision by about 10,000 ambulance workers represented by the Unison, GMB and Unite trade unions to walk out over staff pay and working conditions.

Most ambulance services are set to respond to life-threatening “category one” calls, as well as serious “category two” emergencies. The public have been urged by the NHS to only call 999 in a “medical or mental health emergency” and urged by ministers not to engage in “risky” or dangerous activities.

Barclay has accused unions of making a “conscious choice to inflict harm on patients”. He also criticised them for failing to provide clear national exemptions, similar to provisions by the Royal College of Nursing, who ahead of their strike on Tuesday confirmed that services such as neonatal and paediatric intensive care would go ahead in spite of the industrial action.

“Not all category two calls are being covered by the unions . . . they are being triaged, so we don’t have a national exemption from the trade unions in the way that to be fair the Royal College of Nursing did and that creates significant volatility as a consequence of that,” he told BBC Radio 4’s Today programme.

The health secretary reiterated the government’s decision to accept the guidance of the independent pay body, which had recommended 1m NHS workers on Agenda for Change contracts receive a pay rise of £1,400 backdated to April.

“We’re already three-quarters of the way through this year so what you’d be saying is, go all the way back retrospectively to April to unpick what has been an independent decision by the pay review body,” he told the broadcaster.

He added that next year’s pay review process was already under way. “Obviously that body will then consider the changes in inflation, the other issues that have been raised,” he said.

Speaking on Wednesday, Sharon Graham, Unite general secretary, said that the union was willing to meet the government for pay negotiations “any time, any place, anywhere”, adding that workers had been “treated like dirt” in recent years.

Meanwhile, NHS bosses have continued to voice alarm over the mounting pressures facing the health service, as at least five ambulance services over the past two days have declared critical incidents, a status that is triggered when services are at risk of being overwhelmed by patients.

Matthew Taylor, chief executive of the NHS Confederation, warned that the health service could not “afford to drift into a winter of industrial action”, as he urged unions to work to “minimise patient harm” during today’s strikes.

“In most parts of the country the ambulance service is well away from meeting its targets for responses to those kind of category two cases — so not absolutely urgently, life threatening, but still very important urgent and critical cases,” he told BBC Breakfast.

“So, this strike could not be happening at a worse time because of the pressures the NHS faces,” Taylor added.