Some 200 armed forces personnel are being made available to hospitals across the capital, which has been the epicentre of the Omicron outbreak with a huge upsurge in cases.
The MoD said the deployment includes 40 military medics and 160 general duty personnel to help fill gaps caused by absences due to NHS staff unable to work because they are ill or having to self-isolate.
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They will be deployed in 40 teams of five - comprising one medic and four support personnel - and will be targeted at areas where the need is greatest.
It is expected they will be “on task” for the next three weeks.
In addition, 32 military co-responders are being provided to support the South Central Ambulance Service, working alongside paramedics until the end of March.
Matthew Taylor, chief executive of the NHS Confederation, said the personnel would be “helping out in different ways depending on whether or not they are clinically qualified, so obviously if people have medical skills, then they can be used in clinical settings”.
Others would be used helping “in relation to transport or potentially setting up facilities… we’re building a surge capacity in some of our hospitals to deal with the numbers of patients coming in”.
Mr Taylor said there were several thousand NHS staff absent primarily due to Covid, and that “having two hundred extra people is going to help but it’s only a very small part of what will continue to be a very difficult situation”.
Troops will also be deployed to hospitals in the Midlands in the "coming days", Richard Mitchell, chief executive of University Hospitals of Leicester told ITV News Central on Friday.
Defence Secretary Ben Wallace said: "The men and women of our armed forces are once again stepping up to support their dedicated colleagues in the NHS as they work hand-in-hand to protect the nation from Covid-19.
"They have shown their worth time and again throughout this pandemic, whether driving ambulances, administering vaccines or supporting patients in hospital, and they should be proud of their contribution to this truly national effort."
Armed Forces Minister James Heappy said the government recognises hospitals are currently under "extraordinary pressure".
"This is a time of year that is busy even if there weren't Covid-19. And so put a surge of Covid-19 on top of winter pressures and you have an extraordinary situation that has put the NHS on a war footing," he said.
Armed Forces Minister James Heappy said the NHS is on a 'war footing'
The Royal College of Nursing’s director for England, Patricia Marquis, specified a “staffing crisis” in the NHS.
“The prime minister and others can no longer be dismissive of questions about the ability of NHS staff to deliver safe care,” she said.
“Once the military has been brought in, where does the government turn next in a bid to ‘ride out’ the wave rather than deal with it?”
Around 1,800 service personnel are already deployed across the UK to support the civil authorities in their response to the pandemic.
They include 313 personnel who have been made available to the Welsh Ambulance Service NHS Trust and 96 to the Scottish Ambulance Service.
More than 1,000 service personnel have been provided to support the vaccine booster programme.