Children under 4ft 5in (1.35m) tall could soon have to use a proper child restraint and booster seat in the car or risk a fine for the driver.
Children under 1.35m could be forced to use a booster seat
The new rules are aimed at cutting the estimated 2,000 child deaths and injuries on the roads every year.
The Department of Transport hopes the proposals will be passed by Parliament into law by September.
Road Safety Minister Stephen Ladyman said the changes were for the good of everyone in a car.
"If you are not big enough for an adult restraint then it's not safe for the child and it's not safe for the people in the front seat either, who will be hit in the back of the head by a flying child in an emergency.
"Can you imagine any more horrible an accident than that?" Mr Ladyman said.
Currently, children over three years can use an adult seat belt.
Other changes include the requirement for all children under three to use an appropriate child restraint when travelling in any car or goods vehicle - although rear seats in taxis are exempt if there are no child restraints available.
Rear-facing baby seats must not be used in seats with an active frontal air-bag unless it has been de-activated.
From May 2009, the number of people in the rear of a car must not exceed the number of seat belts fitted.