Most academy schools are achieving greater rates of academic improvement than the schools they replaced, a report has found.
But the National Audit Office warned that academies' performance to date cannot be assumed to be an accurate guide to how the model will perform when expanded to many more schools of different types, as the Government plans.
And it warned that the rapid expansion of the scheme planned by Education Secretary Michael Gove will increase the scale of risks to value for money and will stretch the ability of his department and the Young People's Learning Agency to monitor a growing number of academy schools.
Mr Gove rushed through legislation to allow all schools to convert to independent academies within the state system, and some 142 schools took advantage of it to take on academy status this year, along with 74 approved under the previous Labour administration.
Labour claims that many of the new academies opening under Mr Gove's scheme are more successful secondaries catering to relatively affluent communities, rather than the under-performing schools with disadvantaged pupils targeted under the previous government.
Since 2002, the Department for Education has spent £3.2 billion on the academies programme. This is mainly made up of core funding which would have gone to the schools whether they converted or not, but also includes £288 million in start-up grants to newly-opened academies.
The report found that many of the 203 secondaries which have already taken on the greater freedoms of academy status performed "impressively".
But it cautioned that future academies are "likely to include schools with a much wider range of attainment, and operating in very different community settings", and may not improve their performance in the same way.
Overall, academies increased the rate of improvement in GCSE results when compared with trends in their predecessor schools, found the National Audit Office.
While still below the national average, the proportion of pupils achieving five or more GCSEs at grade C or above was improving at a faster rate than in state schools with similar intakes.