Children are being put in danger because of faulty or badly-fitted car seats.
Safety tests in West Sussex on 292 child seats showed 123 were unsafe.
In most cases the seats were loose or buckles were in the wrong place.
The tests were carried out by trading standards officers from West Sussex County Council in nine towns, including Worthing and East Grinstead, last month.
It follows a similar exercise carried out last year, when 52 per cent of car seats were rated as being dangerous.
West Sussex Cabinet member for public safety Coun Michael Dennis said: "Although the failure rate of 42 per cent is an improvement on last year it is still too high. It would appear that parents are not paying enough attention to in-car safety.
"It is vitally important that people follow the installation instructions and check the fittings from time to time."
Coun Dennis said parents worried about the safety of their child seats can get a free check at a garage operating a Fit safe, Sit Safe scheme.
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