Currently, children weighing as little as 15kg - around three years old - are permitted to travel in backless booster seats, but under the new rules, children below 125cm and 22kg will be banned from travelling in them.
The change in law was originally expected to come into force next month, but the United Nations - which sets the standard for car seats worldwide - must implement the new rules before the European Union can update the law in Europe.
This is now unlikely to happen before March 2017 - and if Britain has left the EU by the the time the ban is implemented, the UK government will update it on its own timetable.
A backless booster seat, also known as a booster cushion, currently satisfies the legal car seats law requirement for children up to 135cm tall, and can be purchased for as little as £6, but is not recommended. However, parents who have already purchased a backless booster seat before the change will be allowed to continue to use them.
But child car seat experts from consumer organisation Which? say that this type of booster seat is unsuitable for such young children.
Which? say a small child is not held as securely in the seat, the adult seat belt is not guided across their body in the best way, and, most importantly, a booster seat offers no protection for a child if your car’s involved in a side-impact crash.
Which? child car seat expert, Lisa Galliers, said: “A decent high-backed booster seat provides better protection in a front crash, as they’re designed to guide the adult seat-belt across the child’s body properly.
“Our crash tests prove they offer much more protection in a side-impact crash than a backless booster seat alone.”
The new rules will only apply to any new products appearing on the market, meaning parents buying a booster seat next year should start to see that they are not approved for use with children under 125cm and 22kg.