The report on the pilot scheme in four local authorities says the Government is misleading parents by talking about nursery vouchers: the scheme is more about four-year-olds starting school early than about nursery education.
Some schools in the pilot authorities have changed their admissions policies to secure more voucher money.
Yesterday, Gillian Shephard, the Secretary of State for Education, said that her officials had written to all local authorities asking them to consider whether reception classes are the right place for young four- year-olds.
Ministers say the pilot has been highly successful with more than nine out of ten parents receiving and redeeming vouchers. In Norfolk, they say, 800 new local authority places have been created, plus 285 private and voluntary ones. Westminster has plans for 1,000 new places in the next three years.
Parents of all four-year-olds will be eligible for pounds 1,100 vouchers from April next year.
Mrs Shephard said: "Vouchers give parents a real choice. They take purchasing power away from bureaucrats and place it squarely in the hands of parents."
But the report from Gillian Pugh, an early childhood expert, says: "There is very little evidence as yet of parents exercising their choice differently from how they would have done before - indeed some are feeling pressurised by schools to send their children to school when they are just four."
Mrs Pugh says the burden of administration for the scheme, which costs just under pounds 10 per voucher, is heavy. "There is a strong feeling of time wasted which could more profitably spent on educating children."
Local authorities in the pilot scheme have been given extra money for administration which will not be available when vouchers are available nationally.
Mrs Pugh's report says it is too early to answer many of the questions about vouchers but points to the fact that while there has been some expansion in the number of places in Norfolk, some playgroups have closed, and a further 22 fear they will.
Yesterday, she criticised the Government's letter urging local authorities to reconsider plans to put four-year-olds in reception classes and to work with private and voluntary groups.
"Ministers cannot have it both ways. If this scheme focuses on parental choice then the marketplace is one in which providers must compete. It is at odds with the spirit of the scheme for the Secretary of State to tell people to work together."
Mrs Shephard announced a pounds 650,000 advertising campaign to publicise the pounds 750m scheme. The helpline set up on Monday has already received 6,000 calls.