Increasing demand for limited nursery places in a competitive environment would mean the most vulnerable children will be penalised, said Gillian Pugh, director of the early childhood unit at the National Children's Bureau.
Dr Pugh was responding to the Audit Commission's report on the education of the under fives. She and other early-years specialists welcomed the report which was published last Friday, but said it highlighted what they had known for years.
The report says high-quality pre-school education depends on chance. In some authorities, more than 90 per cent of three and four-year-olds are in a free nursery or school place, while in others less than 25 per cent get one.
It also warns that the co-operation between state, private and voluntary pre-schools could collapse in the new competitive environment.
Dr Pugh says in the spring edition of the journal Forum: "It seems unlikely that there will be sufficient funding to meet the needs of the one in five children who may require some additional support during their pre-school years.
"There are also concerns that the need to apply for vouchers will discriminate against the most vulnerable families, including those for whom English is not their first language, travellers and families in bed-and-breakfast accommodation.
"It also seems extraordinary that, at a time of financial constraint in the public sector, vouchers are being given to parents who are already paying for, and are prepared to go on paying for, private provision, rather than concentrating resources on the areas of greatest need."