Parents in England are to be given much more choice about when their children start school.
Currently, all local authorities have to ensure all children have a school place by the time they are five.
But most begin part-time, aged four, as "rising fives", with summer-born pupils sometimes offered later starts.
From 2011, all children will be able to start school from the first September after their fourth birthday or take up a free full-time nursery place instead.
The move is being announced just days after a major review of primary education suggested children should not start formal learning until the age of six, whether that is in school or in nurseries.
The government insists the change has been prompted by the findings of its own review of primary education, by Sir Jim Rose, as it relates to summer-born children.
Research considered by Sir Jim suggests children born between July and August are at greater risk of falling behind.
However, there is conflicting research on whether this is due to their being up to a whole year younger than their peers or because they are missing out on a few months' formal schooling because they start later.
Currently, 30 local authorities in England - a fifth - expect summer-born children to start school a term or more later than their peers.
But the change, on which the government is consulting, affects much more than just summer born children.
From 2011, all parents will be able to choose either a full-time school place or a full-time nursery place - if they believe their child is not ready for school.
'Up to parents'
Sir Jim says a smoothed transition from early years to primary schooling, using active play-based learning, would suit summer-born children and those who were still working towards early learning goals linked to the first year of primary school.
The government only expects about half of children to reach these goals by 2011.
Currently all children aged three and four are entitled to a part-time (12.5 hours a week) nursery place, but this entitlement stops once the child reaches school age.
Children's Secretary Ed Balls said: "It is really important that children hit the ground running when they start primary school.
"There is clear evidence the sooner summer-born children start good-quality pre-schooling, the sooner they close the gap on their peers.
"We have put unprecedented investment to expand and strengthen the nursery and early-years sector, introduced the Early Year Foundation Stage so disadvantaged kids do not lose out and are now implementing Sir Jim Rose's reforms to make reception stage exciting and enjoyable for all.
"It is down to parents to make the decisions about when their child starts school."
He added: "We know that not every four-year-old is going to be ready for reception at the same time - so it is important families have the choice when to start full-time or part-time classes or have free early years provision if they want it."
A spokesman for the Department for Children, Schools and Families said it was in negotiations with the Treasury on funding the change.