There is no specific legislation preventing the use of boosters without a backrest. This type of child restraint system is designed in order to be able to adjust the car's 3-point seat belt to the child's. They can start to use a booster once they weigh 15 kg, i.e. when they have outgrown Group 2 (at approximately 4 years old, but this is only a guideline). However, at Fundación MAPFRE we recommend that your child uses a restraint system with a backrest and side protection for as long as possible and certainly while they continue to require a child restraint system (as a minimum up to 135 cm tall, although it is best to keep them in the seat until they are 150 cm tall), as it is safer for the child.
The latest changes brought in have been in terms of approval standards, meaning they affect car manufacturers, who cannot approve boosters without a back rest for Group 2 (children from 15 to 25 kg in weight). This means you will only come across boosters without backrests approved by the R44-04 standard in Group 3.
In addition, booster seats without a backrest cannot be approved under the R-129 standard.
We should take into account that boosters with a backrest help to better position the seat belt and offer increased protection for the child, especially in side impacts. For this reason, it is a good idea to use booster seats with backrests whenever possible and particularly for children under 125 cm tall.