More than half of all car seats fail to protect drivers from whiplash injuries, according to research by Thatcham.
The Motor Insurance Repair Research Centre estimated the problem could be costing insurers up to £400m in unnecessary claims.
Researchers at Thatcham tested 114 different car seats in controlled ‘rear-shunt' simulations.
They rated 36% as poor and 24% as marginal.
Compensation for whiplash injuries costs insurers in excess of £1bn annually. Estimates suggest that claims could be reduced by 30% if the best seats were used and properly adjusted.
Head of the Thatcham test programme, Matthew Avery, said: "These results are very disturbing. Some models were thought to offer proper protection, but when we put them to the test on our dynamic sled, the seats did not perform as expected."
Thatcham is understood to be working closely with most motor manufacturers to improve the quality of seat design.
Avery said: "Almost all manufacturers have changed their seat design in some way thanks to the test. We believe Ford spent £1m redesigning the seats on one of its models.
"Manufacturers are certain to be embarrassed by our findings. They will have to make significant changes if they wish to really protect people in a car crash."