Garages slated after nine out of ten failed to fix car faults that could endanger lives

It also found that 39 per cent of the garages charged for a part that was not supplied.

Which? investigators made four deliberate, easily-fixed faults to 62 cars that a competent mechanic should have spotted, such as a deflated spare tyre, low brake fluid or a blown bulb.

Only eight of the garages returned the cars fault-free while five failed to fix any of the faults.

Which? testers also filled up the cars' screen wash as an honesty test, to see how many garages charged for it. Nearly 40 per cent billed for a refill, with prices ranging from 68p to 2.41.

Which? chief executive Peter Vicary-Smith said: "This is a worrying snapshot of an industry that desperately needs to clean up its act.

"Almost all the garages in our investigation failed to fix basic faults. This could have endangered the lives of drivers and other road users and is simply not acceptable."

Which? said the results also showed that various codes of practice introduced in recent years to improve standards did not guarantee acceptable performance.

Just 39 per cent of the faults were fixed by garages which were part of the Good Garage Scheme. The official Motor Codes scheme, organised by the Society of Motor Manufacturers and Traders, fared better, with 60 per cent of the faults dealt with by affiliated garages.

However, the Bosch Car Service scheme came out best, with 64 per cent of the faults rectified. The figures compared to 43 per cent for non-affiliated garages.

Which? called for a new industry-wide qualification to be introduced for all mechanics, and robust undercover checks on all the codes.

Mr Vicary-Smith said: "The fact the Bosch and Motor Codes schemes have out-performed independent garages is encouraging, but there's still a long way to go for the industry to win the confidence of car owners."

Rachel Greasby, from the Good Garage Service, said she was disappointed with the results of the survey as the faults created by Which? investigators were listed on the industry standard checklist provided to its members.

She said: "Once we have further details on which garages are involved, a full investigation on why they are not using the checklists will be launched and they will be removed or suspended from the scheme as necessary.

"Currently we carry out audits on all members bi-annually unless we have specific reason to assess them before their audit is due. The small minority who fail are removed from the scheme until they meet the required standards."

Motor Codes, which is backed by several government agencies and trading standards officials, said the results showed it had produced improvements.

Director Chris Mason said: "Motor Codes is encouraged that after just two years of operation it is having such a dramatic impact in what is a long-term process of gradual change and improvement,

"We acknowledge there's still a way to go to ensure lasting improvements are made, but this independent research proves that our approach is working."