Some 90% of high street stores that fit children's car seats for free are failing to install them properly, a Which? investigation has found.
The consumer group went undercover across the country to check up on 42 retailers by asking each store to fit two child car seats. Only four got it right.
As many as 13 of the stores failed to ascertain the age or weight of the child and "several" fitted the seats so badly that "a child using them could be in serious danger in a crash", said Which?.
The consumer group said the companies providing the worst seat fittings and which had the highest error rates were Mamas & Papas, Babies R Us and Kiddicare.
Mamas & Papas and Babies R Us only fitted one out of 12 car seats correctly. Kiddicare managed three but one fitting was so bad it warranted its place among the worst three, explained Which?.
Halfords also only got one fitting out of 12 right but the quality of its web-based advice improved its overall performance.
Which? reported that it uncovered a "catalogue of major errors" at Mamas & Papas, the worst fitter overall. It added the findings were especially worrying as the company had previously vowed to improve its fitting service following earlier investigations and warnings in 2011.
The best store was John Lewis, however it still only installed one car seat correctly out of every three fittings.
One common mistake was difficulty in installing 'Isofix connectors', the standard system for all new seats that enables car seats to be attached securely and directly to the frame of a car – which is meant to make installation 'quicker' and 'easier'.
Other errors included "seats fitted with gaps at the back, with the headrest incorrectly still in place, or without being secured tightly enough to the car".
Which? said many of the fitters also gave bad advice and included the example of telling the undercover investigators that the support leg on a car seat base didn't need to touch the floor – something it said was "completely untrue".
All the retailers investigated have been made aware of their shortcomings by Which? and told to improve.
Mamas & Papas said: "At Mamas & Papas the safety of children is of paramount importance and we are extremely committed to car seat safety. Since the report in 2011 we have worked incredibly hard to educate store colleagues and the general public about car seat safety using the Which? charter. The recent customer feedback we have received has been positive so we are disappointed at the results found by Which? following their visit to six stores back in April."
However, the retailer added: "The results of this investigation highlight inconsistencies in advice that is provided to retailers by a number of different bodies."
Meanwhile, Babies R Us, which only managed one safe fitting out of 12, said: "We will continue to build and strengthen our training and support for car seat safety fittings in recognition of the Which? charter, and ensure we continue our commitment to improving these disappointing results."
Halfords added: "We're disappointed with the results of tests taken in April in just six of our stores. We have in excess of 460 stores and have invested in a programme of robust internal training and annual refresher courses for colleagues."
If you're concerned about the safety of your child's car seat, Which? has a 10-point checklist on its website. It points out that common fitting problems include twisted seatbelts, loose harnesses, not all Isofix points being connected and seatbelts sitting incorrectly on your child.