Under a new crisis agreement, firefighters will be able to deliver essential items such as food to vulnerable people, drive ambulances and assist ambulance staff, and collect bodies in the event of mass casualties.
The plan, agreed by the Fire Brigades Union (FBU), fire chiefs and employers, will see firefighters maintain core services such as attending fires and road traffic accidents, but also providing extra services as coronavirus continues to spread.
The plan will run for two months but can be extended if necessary and could affect the UK's 48,000 firefighters and emergency control staff.
Matt Wrack, general secretary of the FBU, said: "We face a public health crisis unparalleled in our lifetimes. The coronavirus outbreak is now a humanitarian emergency and firefighters rightly want help their communities.
"Firefighters are fantastic at teamwork, are experienced in driving emergency vehicles and, as a service rooted in the community, may be best placed to deliver essential items to the most vulnerable.
"Many fear the loss of life in this outbreak could be overwhelming - and firefighters, who often handle terrible situations and incidents, are ready to step in to assist with body retrieval."
Mr Wrack told BBC Radio 4's Today programme it would be "quite a serious challenge" for firefighters to take on more work.
He added: "I think this is a huge challenge across public services and also clearly we need to ensure that firefighters and others are protected in terms of personal protective equipment because no-one can do their job if their own safety is compromised."
It comes as the Metropolitan Police Service and London Ambulance Service urged former workers to return to the service or come out of retirement.
Met Commissioner Cressida Dick is writing to all former officers who retired within the last five years to ask them to rejoin the force, either in a paid or voluntary capacity.
Serving officers who are nearing 30 years' pensionable service are also being asked to delay their retirement.
Ms Dick said: "On behalf of London, and all the men and women of the Met, it is important that we take all reasonable steps to bolster our numbers.
"Demands on us will grow and vary over the coming weeks but I want people to know and see that the Met is here for them."
Meanwhile, the London Ambulance Service tweeted that it was "asking former members of our team to consider returning, if they can, to support us in helping Londoners in need."
It added: "We're particularly keen to hear from former 111 and 999 control room team staff."
Emergency services are among those whose staff have gone off sick with Covid-19 or who have employees in isolation.
As of Tuesday, at least 280 workers in the London Fire Brigade were in isolation (5 per cent of its overall staff), while the West Midlands Fire Service, which covers Birmingham, had 105 staff in isolation (5.5 per cent of staff) and the Scottish Fire and Rescue Service had 285 staff in isolation, according to the FBU.