Firefighters are to be dispatched to medical emergencies to help London paramedics cope with an “unprecedented” number of 999 calls.

A pilot scheme in four London boroughs will see fire crews deployed at the same time as ambulances to patients in life-threatening situations.

Firefighters will provide basic life support and use automatic defibrillators to restart hearts if they arrive first, but will not take patients to hospital.

It comes as London Ambulance Service revealed that last month was busier than December for the first time, with January having 46,201 “Category A” emergencies, the highest number ever recorded and almost 1,000 more than the previous month.

LAS chief executive Dr Fionna Moore said it had received more than 300 calls an hour “on a regular basis” over the last fortnight – well above the normal rate of 250 an hour – in the latest indication of the soaring pressure on the NHS.

The busiest period was 2am to 3am on January 1, when LAS received 512 emergency calls, compared to 469 last year. “It’s unprecedented, what we are seeing,” LAS director of performance Paul Woodrow said.

The “co-responder” partnership with London Fire Brigade will run for four months in Merton, Wandsworth, Newham and Lambeth, and elsewhere in the country, after the Fire Brigades Union withdrew long-standing objections.

The pilot will capitalise on the lower “utilisation” rates for firefighters, who spend less of their time on calls than ambulance crews.

Metropolitan police vehicles in Enfield, Croydon and Ealing began carrying defibrillators last November in a similar initiative to improve survival rates for people suffering out-of-hospital cardiac arrest.

The LAS has also partnered with the Good Sam phone app, which directs trained life-savers, such as doctors and nurses, who are off duty but have volunteered to help, to incidents nearby.

Ambulance chiefs said there had been a 20 per cent increase in the most serious “Category A” calls, which include cardiac arrest, suspected heart attack and other immediately life-threatening conditions, in the last two years.

Many of the additional calls relate to patients with breathing difficulties, an indicator of how the NHS is having to treat an older and sicker population.

Jill Patterson, LAS interim director of performance, said: “Across 2013/14 we were seeing 1,200 Category A calls daily. At the moment we are seeing just short of 1,500 daily. That is an increase of about 20 per cent.”

Crews reached 66 per cent of incidents within eight minutes – well below the 75 per cent target.

However “green shoots” of recovery were reported, with performance on four days in December exceeding target, despite staff shortages and the huge number of calls. LAS crews attended 92,248 calls in December, the highest figure of 2015 and more than 4,000 up on November.

LAS interim medical director Dr Fenella Wrigley said: “We have seen an increase in viral illnesses like coughs and colds and more people suffering with respiratory illnesses – in particular more elderly people and people with underlying medical conditions such as asthma or lung disease who may be at greater risk of complications like chest infections.”