The brakes on a third of all cars could fail without notice, according to the results of tests on nearly 700 vehicles carried out by independent service garages on behalf of the car care products company Comma. They found one in two motorists had defective braking systems, with 29% of them so poor they were unintentionally risking their lives every day.

Over time brake fluid absorbs water from the atmosphere, which reduces its effectiveness by lowering the boiling point. Test equipment supplier Alba Diagnostics recommends replacing defective fluid when it begins to boil at 200 degrees Celsius. Anything below 180 degrees Celsius is said to be potentially lethal but some of the cars tested by Comma were as low as 100 degrees ¨C the same as water.

Researchers found that the problem is not necessarily linked to the age and mileage of vehicles; one in five vehicles with 120,000 miles or more on the clock showed brake fluid boiling temperatures of below 180 degrees.

If brakes are tested mechanically after an accident when the fluid has cooled down, braking function will appear normal.

Acting Chief Inspector Mick Turner, of the Collision Investigation Unit at the Metropolitan Police, said: "We see too many victims of road accidents every year who have suffered a crash simply as a result of not maintaining their car properly. It's important that motorists remember to maintain every part of their car's safe running - that is, after all, why car manufacturers recommend set regular service intervals. Clearly, the onus is on the driver to make sure their car is safe, and that includes aspects of their car that they might not normally think to check, such as brake fluid."