Experts have called for the regulation of car air conditioning systems after new trials revealed that popular models are letting in dangerous amounts of pollution.

Safety tests on 11 types of car have shown huge variation the the ability to purify incoming air, exposing those inside to millions of poisonous particles with each breath.

The Toyota C-HR performed worst, blocking out just 1 per cent of pollutants, while the VW Polo managed only 35 per cent and the Ford Fiesta 40 per cent.

However, the Mercedes E-Class was able to purify 90 per cent of the incoming air, demonstrating that the technology exists to substantially protect passengers.

Emission Analytics, the laboratory which carried out the tests, said the disparity was due to an absence of Government standards regulating air filtration systems.

Nick Molden, its chief executive, told The Sunday Times: “Drivers can be exposed to high pollution levels while believing themselves to be protected by the air filtration and ventilation system.”