The move to a September start for all four-year-olds next year will cost the education budget half a billion pounds over the next decade, a government impact assessment has concluded.
The proposal was the most controversial part of the review of the primary curriculum carried out by Sir Jim Rose, and has been opposed by almost two-thirds of people in a consultation last term.
The official estimate for the cost is between #163;43 million and #163;57 million a year. The analysis of the policy estimates that around 7,300 children would take up the offer of an early start. It estimates the 10-year cost to government would be #163;481.2 million, which will be paid to authorities to cover the extra places.
Most authorities already admit children in the September after their fourth birthday, but about 40 let younger classmates begin in January. It is believed that 11 also have an April admission date for summer-born children.
The move is designed to provide a level playing field, allowing parents across the country to have the option of a full-time reception place.
Margaret Morrissey, of Parents Aloud, said: "Legally it is an option but in practice it's really not, because schools cannot afford to hold the place.
"I can't see the point in the Government having a consultation, getting an answer and then carrying on regardless. It makes a mockery of it."
After growing pressure from early years experts and parents, Ed Balls, Schools Secretary, made it clear that parents would be allowed to defer a reception place until January or April and the Government would fund the equivalent hours in a private or other nursery until the child was ready to start school.
The Government launched a four-week consultation in October on the proposal consisting of just one question - do you agree with the proposal?
There were 489 responses to the question - 32 per cent agreed, 62 per cent disagreed and 7 per cent were not sure.
More than half the respondents were heads, and their main concern was funding if parents decide to admit children part time or defer entry.
Parents were concerned that there was no real choice as they would feel pressured into sending their child early, and many felt that four was too young.
The analysis mentioned that other parental surveys suggested that 61 per cent of parents with children aged four to 10 agreed that children should start school in September after they turn four - including 63 per cent of parents with summer-born children.
Local authorities welcomed the proposal, saying it would prevent confusion when parents moved.