Around 12% of firefighters and control room staff in some areas are self-isolating during the coronavirus pandemic, says the firefighters' union.
The Fire Brigades Union (FBU) says nearly 3,000 fire and rescue staff across the UK are in isolation.
It has called on the government to provide urgent coronavirus testing of its members so they can return to work.
A government spokesman said it is working with fire chiefs to ensure they have the support they need.
Matt Wrack, FBU general secretary, warned services would be put on a "dangerous knife edge" if staff cannot be tested and have to isolate unnecessarily.
The union warns that without urgent testing of front-line staff there will be an "inevitable" impact on fire brigades' ability to respond to emergencies, and assist with the coronavirus outbreak.
Some firefighters have been driving ambulances; delivering food and medicines to vulnerable people and helping police move bodies.
They will also now fit masks and respirators for NHS staff and deliver protective equipment and other medical supplies.
The union says responding to emergencies and helping with the UK's coronavirus response places fire staff at a "greater risk of infection".
It says England is the only nation of the UK not to make any commitment to test its members for Covid-19 - in Northern Ireland 50 firefighters with symptoms have been tested, and there are programmes planned in Scotland and Wales.
Initial limiting testing of firefighters and control room staff has already begun at Glasgow Airport, according to the FBU.
Of the 3,000 fire and rescue staff in isolation, 2,600 are firefighters or work in control rooms.
In Bedfordshire, 12% of staff are off work - the highest proportion in the country.
In London, 10% are self-isolating, which is equal to 472 firefighters or controllers.
Meanwhile, West Yorkshire is short of nearly 16% of its control room staff.