Firefighters are expected to respond to around 28 life-threatening calls a week as part of a co-response trial.

The four month pilot will see fire crews, who already carry defibrillators, to respond to emergencies until a paramedic arrives on scene.

It begins in Newham and Merton today and will be rolled out in Wandsworth and Lambeth on Wednesday next week.

The initiative is aimed at meeting the demand faced by ambulance crews across the capital.

Chris Hartley-Sharpe, head of first responders at the London Ambulance Service said: “Our ambulance crews are facing unprecedented demand, treating over 1,500 critically ill patients every day and a further 2,000 patients with less serious illnesses and injuries.

“This initiative is a fantastic opportunity for the emergency services to work together and share resources and help save even more lives across London.”

Under the trial, if a 999 call meets a set criteria, a crew from each service will be dispatched at the same time.

If the London Fire Brigade crew is able to get there faster, they will start basic life support until ambulance staff arrive.

If the ambulance arrives first, the Brigade will support them until the patient is taken to hospital.

The criteria for calls which will be co-responded to covers patients whose condition is immediately life-threatening as a result of cardiac or respiratory arrest.

Gareth Bacon, the chairman of the London Fire and Emergency Planning Authority, which runs the London Fire Brigade said: “As the number of fires and fire deaths continue to decline across London I’m delighted that firefighters can find new ways to use their excellent training and remarkable skill to help save even more lives.

“When it comes to treating someone in cardiac arrest, if firefighters are closest, it makes perfect sense that they should respond to improve the survival chances of those in need of rapid help.”

The London Ambulance Service is the busiest in the UK and has already launched a number of initiatives to increase cardiac arrest survival rates.

Defibrillators have been added to 110 police vehicles while over 1,000 were given to shops, gyms and businesses in the capital last year.