The rapid expansion of England's academies programme risks being poor value for money, the spending watchdog has warned.

With one in four existing academies needing extra financial help, the National Audit Office said much tighter monitoring was needed.

It also found poorer pupils did less well in academies than in regular state schools.

The government said academies performed well and that it was reducing risks.

The NAO said these state-funded, privately-run schools begun in disadvantaged areas by Labour had performed impressively in their efforts on improving attainment.

But since coming to power, the coalition government has encouraged schools rated outstanding by Ofsted to apply for academy status, saying they would be fast-tracked.