London is to have no air ambulance for the next three weeks as the capital's one and only emergency helicopter is going in for maintenance.

It might be surprising considering London is one of the most affluent cities in the world, but there is only one air ambulance for reaching critically injured patients that covers the capital's population of nearly nine million people, who are living and working within the M25 (a 600 square mile radius).

The London's Air Ambulance charity runs a 24/7 advanced trauma care service that includes a helicopter flown by two pilots during daylight hours based on the roof of Royal London Hospital in Whitechapel, as well as rapid response cars that work by night.

Since its launch in 1989, the air ambulance service has treated over 32,000 patients and has achieved world-leading survival rates of 18% for traumatic cardiac arrest and pre-hospital thoracotomy, as well as some of the world's highest success rates for adult and paediatric intubation.

The charity was also the first in the world to perform pre-hospital Resuscitative Endovascular Balloon Occlusion of the Aorta (REBOA) surgery to prevent a trauma patient from bleeding to death.

In previous years, the charity was able to rent a relief MD902 Explorer helicopter for the 18-day annual maintenance period, however, due to changes in European regulations regarding multi-pilot rules, the charity has been unable to find an appropriately equipped helicopter this year.

It costs £5m ($7.6m) a year to run London's Air Ambulance and the charity treats an average of six seriously injured people a day.

The charity receives £1.3m as a "benefit in kind" from the NHS. Barts Health NHS Trust pays for the salaries of the trauma doctors who work on the trauma care service, while the paramedics are paid by the charity.

All additional funds have to be raised from public and corporate donations.

"When seconds count, the most effective way to reach critically injured patients is by helicopter," a spokesperson for the charity told IBTimes UK.

"At the end of January we are launching a public appeal to raise the remaining funds needed to secure a second aircraft to ensure there is always a helicopter available for London in daylight hours."

So far, the charity has been given £1m by Chancellor George Osborne towards the new helicopter and corporate donors including BlackRock have raised another £700,000.

If the helicopter could be up and running by the summer, it would be able to extend daylight flying by up to 16 hours and the service would be able to reach an additional 400 patients per year.