1,200 troops to cover for ambulances and Border ForceAbout 1,200 members of the military and 1,000 civil servants are to be drafted in to cover for striking ambulance and Border Force staff over Christmas.
The government say the personnel will plug staffing gaps and keep front-line services running.
Some 10,000 ambulance staff in England and Wales will strike on 21 and 28 December in a pay dispute.
Unions say military staff are not "sufficiently trained" to taking on ambulance roles.
But Health Secretary Steve Barclay said his "number one priority" is keeping patients safe.
However he declined to discuss "political debates", stressing that the military is directed by the government to "serve the nation".
The head of the armed forces has warned against viewing troops as "the go-to" option when it comes to covering strike action.
Speaking to the Sunday Telegraph, Chief of Defence Staff, Admiral Sir Tony Radakin, said: "We're not spare capacity. We're busy and we're doing lots of things on behalf of the nation - we've got to focus on our primary role.".
The Welsh government has said the military will not be asked to drive ambulances in Wales.
The co-ordinated walkout was called by the three main ambulance unions - Unison, GMB and Unite.
All three are taking action on 21 December. GMB union members will go on strike again on 28 December.
The government said pay rises for ambulance workers and nurses were decided by independent pay review bodies - which have recommended pay rises of around 4.75%.
With inflation running at more than 10% and many struggling with the rising cost of living, unions have said this represents an unaffordable real terms pay cut.
The ambulance strikes will affect non-life threatening calls only. Hospitals have been warned by NHS bosses to prepare for "extensive disruption" during the walkouts, and to free up beds to make it easier to move people through A&E and get paramedics quickly back on the road.
However, doctors' leaders warned this would be very difficult to achieve.
The government said the health service will put into place "tried and tested" plans to reduced risks to patient safety and disruption, while individual NHS trusts will work with unions to agree on a safe level of cover.
But chief executive of the NHS Confederation Matthew Taylor said there was "no question" strikes would pose risks to patients.
"We're in the middle of winter and we have a health service which, even on an ordinary day without industrial action, is finding it difficult to cope," he said. "So there are going to be risks to patients. There's no question about that."
Troops from the Army, Navy and RAF will be deployed. Some 600 members of the armed forces are due to take on ambulance driving roles and 150 personnel will provide logistical support.
Community first responders will also be used to help manage demand for medical care.
Mr Barclay described the decision to go ahead with strikes as "deeply regrettable", repeating the government's position that the striking unions' demands are "not affordable" - but he also said that he was "open to further discussions".
"My number one priority is to keep patients as safe as possible and we are stepping up preparations across government and the NHS, including making best use of the armed forces, volunteers and freeing up capacity to mitigate disruption and ensure safe staffing levels," he added.