Parents armed with nursery vouchers are forcing education authorities to review school admission arrangements and consider taking children as soon as they are four.
Kent will be holding meetings with primary schools in the summer to discuss the problem, and Bedfordshire is reviewing its admission policy to avoid paying tiwce for some children's education.
Lack of classroom space could thwart demand for schools to take pupils as soon as they become eligible for the voucher at the age of four.
Michael Edwards, county education officer for Norfolk, told a conference that parents faced with vouchers and uncertainty put undue pressure on schools to admit children earlier. Norfolk - one of four authorities taking part in the pilot scheme - had decided that schools could only take children earlier if they met the county's space, equipment and staffing requirements.
Kent is debating four options: to admit children in September and January as usual; to encourage schools to admit all children in September; to encourage schools to admit children in the term after their fourth birthday; or to leave schools to make arrangements.
Roger Crouch, head of education policy for Kent, says in a briefing paper for the county's education committee: "The educational and financial implications of each of these options is complex... if the voucher scheme leads to the admission of the youngest four-year-olds into reception classes in response to the vouchers stimulating demand, then that will have significant implications for classroom management and teaching and learning within the primary school. There may also be significant pressure on space and class size."
Keith Fossey, an education manager for Bedfordshire, said: "We believe many schools will want to benefit from vouchers by taking children as soon as they are four but to do that quite a few will need more accommodation. We are looking at ways of meeting these needs."
Bedfordshire treats children who become five between September 1 and the first day of term, or between January 1-15 and April 1-30, as statutory fives and funds them accordingly. Under the Government's scheme, said Mr Fossey, such children also qualify for vouchers, with the potential for double-funding - estimated at Pounds 300,000 in Bedfordshire's case.
The National Children's Bureau, which is evaluating phase one of the nursery scheme, says the three London boroughs in the pilot, Wandsworth, Westminster and Kensington and Chelsea, have decided they want to keep their existing admission arrangements.