The coolant for car mobile air conditioning systems, called HFO-1234yf, has been approved for use in the US, Japan and Europe and Toyota and Suburu have started fitting their cars with the substance.

It was chosen as it is produces 98 percent less climate damaging that its predecessor, R134a.

However, a German expert on the compound has warned that it is should be scrapped as it is much more flammable than the current coolant and when heated above 500C (932F) releases hydrogen fluoride, a highly toxic gas. Temperatures in car fires can easily reach twice that heat.

“You have 600g of this cooling agent per car, which if it burned completely would produce 200g of hydrogen fluoride at a level of concentration that is very high. For a human just one gram is deadly – either inhaled in gas form, through the skin or when dissolved in water,” warned Prof Andreas Kornath, an inorganic chemistry professor at Munich University who has been studying the substance for 20 years.

The odourless gas has no instant effect but once inside the body a person dies within a day or so in terrible pain due to internal burns and muscle failure.

“This product should not be on the market. There is a real risk every time a car catches fire, which happens 30,000 times per year in Germany alone,” he told the Daily Telegraph.

Prof Kornath issued his warning at the European Parliament in Strasbourg.

Honeywell, which produces the product, has refuted the claims.

“The risk of HF formation is not higher than with R134a – and this refrigerant has been used for decades without any recorded incident,” said Honeywell’s Sabine Chmielewski.

SAE International Cooperative Research Program, which comprises leading automakers, found HFO-1234yf to offer “superior environmental performance” to CO2 while having “the lowest risk for use in mobile air conditioning systems in meeting environmental and consumer needs.”

But Prof Kornath alleged the product had been mainly approved on the basis of tests compiled by the chemical makers, not the findings of independent research.

BAM, Germany’s Federal Institute for Materials Research and Testing, for instance, had warned of the risks of using it, he said.

He recommended using carbon dioxide as a coolant, as the fire risks and toxic hazards were nil.

Toyota confirmed it would start using the new refrigerant in new models starting this year.

The car maker’s spokesman Jean-Yves Jault said: “We think the new refrigerant is as safe, yet much more environmentally friendly, as the previous one. This was confirmed by the SAE CRP investigation whose outcome we support.”

“We undertook flammability tests and risk assessment with an independent third party institute, and the results confirmed the safety of the new refrigerant.”

He said that that the phenomenon of hydrogen fluoride gas is “not new”, and that concentration of fluorine atoms was “actually much higher” in the historic refrigerant, R134a.