Some are offering new parttime places in reception classes to three-year- olds who are not eligible for voucher money, so that they can start recouping the money the moment a child reaches four. The legal starting age is five.
The changes are controversial, because early-years experts say many reception classes do not have the staff or equipment to cater for very young children. Starting school too soon, they say, may put children off education for life.
Under the Government's scheme, all four-year-olds receive pounds 1,100-a-year nursery vouchers from 1 April.
Private, voluntary and state nurseries have to compete for the money.
Early-years campaigners say they fear that the voucher scheme may force English local authorities to follow the Welsh example. Statistics from the Department for Education show there are already 1,416 three-year-olds in school reception classes.
The voucher scheme has led to a new school starting date of just four rather than rising five, or five, in many parts of England, as local authorities try to maximise their voucher income.
In Wales, Geraint Elis, Anglesey's Assistant Director of Education, said by September, 35 of the council's 51 primary schools would be admitting children part time after they reached their third birthday.
Mr Elis said: "We need to recoup as much money from the voucher scheme as we can. We are not happy about admitting children each term, so we are reducing the admission age by a full year."
All reception classes, he added, had a nursery assistant as well as a teacher and the number in a class would be limited. Nearby Gwynedd is also changing its admissions policy to admit children to school part-time at three.
Powys County Council is offering a full-time reception class place to rising fours from this term if schools have enough staff and equipment to cope with them. A spokesman for the council's education department said the change in admissions policy from the date that vouchers became available was "coincidental".
It followed a survey of heads who had mostly favoured the change. Those who felt children were too young to start school at three would continue to admit them at four. Wendy Hawkins of the Wales Preschool Playgroups Association, said: "Teachers are bullying parents into sending children to school too early.
"These children are often put into inappropriate settings in small village schools in classes with a wide age range of children. Local authorities have panicked because they fear children will go elsewhere."
Welsh children have traditionally started school earlier than their English counterparts and there are already many rising fours in Welsh schools. Both countries have an earlier starting date than other western European countries.
Margaret Lochrie, of the Preschool Learning Alliance, said: "The legal age for starting school seems to mean very little. It is a concern to us that the early move into school won't stop at four."