In a poll to mark the launch of Child Safety Week, Which? showed that 17 per cent did not use a car seat at all.
A further 30 per cent used backless booster cushions which, though they meet legal requirements, offer less protection than full-sized child car seats in side-on crashes.
Which? chief executive Peter Vicary-Smith said: "Every year in the UK, around 30 children under 12 years old are killed while travelling in cars, and a further 300 are seriously injured. Kids might pile the pressure on parents not to have to sit in a full-sized car seat when they get a bit older, but it could mean the difference between life and death."
He went on: "Nobody who has seen the footage of a side-impact collision on our website would choose to use a backless booster seat.
"While they're better than using no car seat at all, they simply don't provide enough protection."
Which? is calling on manufacturers to phase out backless boosters even before the legal standards are changed.