New schools are being built to higher specifications
The government faces "a real challenge" to complete its flagship school building programme on time and on budget, the National Audit Office says.
It says Building Schools for the Future is two years late and, at £52bn-£55bn, up to £10bn over budget.
The government aims to revamp or rebuild 250 schools a year to meet its aim of refurbishing all 3,500 secondaries in England by 2020.
It insists the programme is back on track after initial difficulties.
The delays have been put down to complex procurement deals and local authority difficulties getting to grips with the new scheme.
It was unclear how much problems in the finance markets will affect the programme.
The head of the Commons public accounts select committee, Edward Leigh, said ministers' ambitions were "fanciful".
Mr Leigh said the programme had been "beset by optimism from the outset".
Timelapse footage of one school's novel apporach to building a new gym
The Department for Children, Schools and Families had set unrealistic targets, underestimating delivery times and cost.
"It should come as no surprise therefore, that the programme is behind schedule and its cost is burgeoning," he said.
"Four years into the programme, only 42 schools have opened, yet the department is confidently predicting that it will manage to open 250 schools per year after 2011.
"Given the rate of progress so far, this seems fanciful."
It has long been acknowledged by the government that BSF, as the programme is known, got off to a shaky start.
The head of the National Audit Office, Tim Burr, said its management had improved.
This is an unprecedented project, never attempted before by any other country - it is not a race to spend the money as quickly as possible
Schools Minister Jim Knight
"But it remains a real challenge, in difficult market conditions, to deliver the 250 schools a year that will be needed, to include all schools by 2020 as currently planned."
The audit office said the local authorities and companies involved believe that it was leading to more strategic procurement of school infrastructure than previous school building programmes.
BSF schools were built to higher specifications and space standards than previous schools yet costs were in line.
The chief executive of Partnerships for Schools, the organisation set up to run BSF, Tim Byles, said that in these uncertain economic times, the audit office had recognised that BSF was less susceptible to market conditions than other large scale infrastructure projects.
"With 50 schools now open, we are ahead of our delivery schedule for the current financial year, and to date, no construction works or school openings have been delayed due to the current economic climate."
Schools Minister Jim Knight said: "BSF is building momentum - with 50 schools now open and 200 opening a year by 2011.
"This is an unprecedented project, never attempted before by any other country - it is not a race to spend the money as quickly as possible.
"We have always been upfront about the early delays but we've listened and learnt lessons. We want value for every single penny of taxpayer's money."
Mr Knight added: "If nothing else [by 2020] every secondary school will either have been rebuilt or be in the process of being re-built, with hard-hats on site."
Yorkshire PostSchool refit plan 10bn over budget The Government's ambitious programme to rebuild or refurbish every secondary school in England is up to 10 billion over budget and almost two years behind schedule, a damning report has concluded. - 34 hrs ago
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.