Parents are getting "incorrect and potentially dangerous advice from retailers" on how to fit child car seats, according to Which?
Mystery shoppers visited seven branches of each of the five major child car seat retailers.
No more than two of the seven stores from each of Halfords, Mothercare, John Lewis, Mamas & Papas or Babies R Us gave the correct advice when it came to correctly installing the seat, Which? said.
It added that no retailer had more than three of their visited stores asking what type of car the seat would be used in - a vital question.
Mothercare came top of the five chains investigated but still scored much lower in comparison to independent stores Which? tested last year.
Which? added that Mamas & Papas and Babies R Us had made a substantial improvement but were still a long way from satisfactory, and Halfords and John Lewis scores were "worse than in our previous investigation".
None of the five retailers improved on their scores for correctly installing a child car seat, compared with the previous investigation in October 2011, with three doing worse.
Which? executive director Richard Lloyd said: "It is unacceptable that major high street retailers are still giving poor advice and failing to highlight crucial information that is key to a child's safety, despite them promising to do better last year.
"We want retailers to improve their staff training and knowledge of child car seats so that all major high street retailers offer the correct advice every time.
"Parents should be able to trust the advice they get from major retailers."