Mr Robertson insisted that the independent evaluation, to be headed by Professor Sally Brown, would not conceal any concerns raised. He stated: "We are determined that the voucher system should deliver the maximum benefits for its ultimate consumers, pre-school children, and the evaluation will be a crucial tool in ensuring that that happens."
Mr Robertson added: "We will share the results widely. We have arranged with the Stirling University team that it will provide not just a final report but interim reports and not just interim reports but dissemination events at which the emerging findings can be reported and discussed."
The study would be carried out in tandem with local authority evaluations, and the Convention of Scottish Local Authorities would nominate members of the evaluation advisory committee. A survey of parental views will be included.
Mr Robertson said vouchers were making clear progress, with 3,200 parents applying. "There has been a massive expansion of provision in the pilot areas," he added.
Helen Liddell, Labour's education spokeswoman, replied that the Scottish Office had ducked issues of quality assurance and that parents, teachers and the Scottish public had rejected the voucher scheme. "I make no apology for repeating again and again that a nursery voucher is not a guarantee of a nursery place. It will flaw the provision of pre-five education in Scotland, " Mrs Liddell said.
Ian Davidson, Labour MP for Govan and a former education convener in Strathclyde, appealed for far greater spending on pre-five education in areas of need, especially on children with lone parents. "Regrettably the existing pattern of pre-school education is not designed to help parents, particularly single parents, find employment," Mr Davidson said. "The hours are short and the holidays are long. Pre-five provision should be family friendly, should be longer, should be flexible and should have a longer operating year."
Jim Wallace, the Liberal Democrats' Scottish leader, said the scheme would not deliver guaranteed nursery places. Many parents would be unable to afford top-up costs and Mr Wallace feared an emphasis on four-year-olds would lead to fewer places for three-year-olds.
Andrew Welsh, SNP member for Angus East, accused ministers of ignoring parental choice. "When parents are consulted about nursery education they do not choose these Tory vouchers," Mr Welsh said.