The brakes on a third of all cars could fail without notice, a new report warns.

Tests carried out by independent service garages on behalf of car care company Comma found half of all motorists had defective braking systems, with 29% of them so poor they were risking their lives every day.

Quality of the brake fluid was checked on around 700 vehicles. Over time, brake fluid absorbs water from the atmosphere, which reduces its effectiveness by lowering its boiling point. Brake fluid testing experts recommend replacing defective fluid when it can boil at 200 degrees Celsius, and that any fluid that boils at below 180 degrees is potentially lethal. Worryingly, some cars tested had the same boiling point as pure water.

At one of the testing stations, 70% of vehicles were defective and 44% of those were classified as unroadworthy. Researchers found that age and mileage of the vehicle were not necessarily a factor, with a fifth of vehicles with 120,000 miles or more on the clock showing boiling temperatures of below 180 degrees.

“One minute your brakes are working, the next they aren’t”, said Comma’s Mike Bewsey. “Once your brake fluid is contaminated, there is a much greater risk that it will boil. This can happen under stop-start conditions, heavy braking or towing. There is no warning, and once the brake fluid has cooled down again braking function appears normal.”

Comma believes the research, the most comprehensive to date on the issue, supports the theory that poor brake fluid could be responsible for thousands of serious accidents every year.

Bewsey added: “There is a real lack of knowledge amongst drivers of the importance of brake fluid, and how it works. Brake fluid is a safety critical item and it’s important that this is recognised and its boiling point tested on a regular basis”.