An undercover investigation by Which? found that 90 per cent of the stores it visited failed to fit the car seats correctly.
Experts visited 42 stores across the UK and asked staff to fit two different car seats, with just four managing to fit both properly while 13 failed to ask any essential questions beforehand.
The watchdog said the retailers Babies R Us and Mamas & Papas each failed 11 of the 12 fittings with "a catalogue of mistakes" while Kiddicare failed nine.
Some of the errors included fitting the seat with the support leg still folded under the base of the seat and one seat not attached to the car securely enough because the fitter did not know how to install it properly using the seat belt.
However, Which? said all the stores it visited had room to improve, with Halfords managing just two successful fittings, Mothercare three and John Lewis, the best of the major retailers, failing two thirds of its installations.
Independent stores did slightly better and achieved the most successful fittings with seven out of 12, but the Which? experts thought their advice was not as comprehensive as it should have been.
The consumer group said the majority of staff it saw did not spend enough time fitting and explaining how to use the car seats.
Mamas & Papas told Which? it had worked "incredibly hard" to educate store colleagues about car seat safety. Babies R Us said it would build and strengthen training.
John Lewis said car seat fitting was mandatory training and Halfords said it had accredited fitting courses with annual refreshers.