Fire brigades are stepping in because of a surge in demand caused by the ageing population and difficulties in making GP appointments. Ambulance services are also struggling to recruit paramedics.
At the same time, calls to the fire service have fallen because of major advances in prevention and safety.
The 45 brigades in England have adopted 'co-responding', whereby firemen are routinely sent to ambulance calls. This could mean either a single fireman in a car or a crew of four with an engine.
Figures from the Home Office show that firemen were sent to 44,121 ambulance calls in the 12 months to June 2017. In 2010, the number stood at just 10,329.
The practice has raised concerns that firemen are being used to 'stop the clock', and ensure ambulance services hit response time targets.
In Kent, records from the end of November show that fire crews were sent to between nine and 16 medical calls a day.
Incidents included unconsciousness, breathing difficulties, back pain, fitting and cardiac arrests – where the heart has stopped beating.
A whistleblower, who works for Kent Fire and Rescue, claimed firemen did not have the qualifications or the training to treat the majority of patients.
He said staff did six days of basic first aid – including how to use a defibrillator – with a three-day refresher course every three years.
He added: 'It's just so wrong. It's putting people at risk of fires. If a pump is off the road, and there's a house fire nearby, then there's no one to attend it for far too long. We only have a certain amount of kit.
'We have a first aid bag and it's got tourniquets, oxygen, bandages to stop haemorrhaging and a defibrillator. But we have no drugs whatsoever. No adrenaline which is what they need.'
The Fire Brigades Union declared following a pay row that crew members would no longer be obliged to respond to medical calls.
But many are still choosing to go out, stopping the rest of their crew from responding to fires. By law, engines – or 'pumps' – must be manned by at least four firemen to be operational.
Lib Dem health spokesman Judith Jolly said: 'Turning our firefighters into paramedics is a dangerous and slippery slope. Fire services have an important role to play responding to some medical emergencies, but shouldn't be relied upon as a matter of routine.'
Dave Green, national officer for the Fire Brigades Union, said: 'Words fail me as to the way this country deals with emergency medical response.'
A spokesman for the Association of Ambulance Chief Executives said firemen were used 'alongside an ambulance resource and never instead of an ambulance resource'.
A spokesman for Kent Fire and Rescue Service said: 'KFRS has been working closely with South East Coast Ambulance Service responding to medical emergencies since 2003.
'KFRS always prioritises its response where there is a significant risk to life, and when required resources are relocated from other areas of the county to respond to and to support an increase in demand.'
A Home Office spokesman said: 'Collaboration across our emergency services presents a real opportunity to maximise resources, enhance local resilience and improve the service.'
When ambulance service telephone handlers receive a 999 call, they establish how serious the incident is and where the nearest ambulances are.
If a fire station is nearer than the closest ambulance – and it is judged to be an emergency – the fire service will be asked to send a first responder.
Experts have repeatedly warned that ambulance services send unsuitable staff simply to hit strict response time targets.
'Community first responders' – some armed with little more training than a first aid certificate – have been used to make sure services meet the eight-minute target for the most serious calls.
NHS England changed the rules in July so the clock stops only when an ambulance – not a car or bike – arrives. It also shortened the target to seven minutes while extending it for less serious injuries.