A recent study has cast doubts over the Conservatives' school reform policies by showing that more than a third of charter schools in the United States are giving pupils a "significantly worse" education than standard state schools.
The report by academics at Stanford University reveals that only a fraction of charter schools have shown any improvement. Their study throws a shadow over Tory policies, which draw heavily from the US charter system. If they win the next election, the Tories plan a proliferation of what they call New Academies, a blend of the charter and Swedish free schools.
The Stanford study found that 37 per cent of the schools were providing a worse level of education than standard state schools. Only 17 per cent of them delivered a better education, while 46 per cent made little or no difference.
It followed a warning last week from Arne Duncan, the US secretary of education, that the charter school movement was "putting itself at risk" by allowing substandard providers to set up schools.
Campaigners against an increase in the number of academies in England said the news was further evidence that the quasi-independent schools should be scrapped.
Pete Jackson, spokesman for the Anti Academies Alliance, said the Stanford research showed that only a very small number of pupils would see any educational benefit from attending the schools.
"If 37 per cent of charter schools are doing worse and nearly 50 per cent are showing no improvement, then a very small number are actually improving education," he said. "It is far fewer than what people were led to believe.
"The recent (London School of Economics) Centre for Economic Performance report suggested that academies were only doing as well as their neighbouring schools, and this US report further undermines the basic tenet of academies, which is that they are better than normal schools. It's simply not true."
But the Tories are sticking to the plan, which would see them divert Pounds 4.5 billion intended for Building Schools for the Future to develop hundreds of the New Academies. Michael Gove, the shadow schools secretary, pointed to poor educational structures in states that were failing to make any progress as the reason for the Stanford findings.
"There are many studies that show that choice and charter schools can bring a dramatic improvement," he said, "which is why President Obama supports them.
Jay Altman, chief executive of First Line Schools, a charter school operator in New Orleans, is unsurprised by variations in performance uncovered by the Stanford study. "We are learning there are certain regulatory mechanisms that facilitate a high degree of consistency," said Mr Altman, who was in England this week to speak for Future Leaders, a training programme for heads of urban schools that he helped develop in England.
Following Hurricane Katrina in 2005, more than half of publicly funded schools in New Orleans became charter schools. Critics described this as an "educational landgrab" by capitalists exploiting a disaster that would depress teachers' wages because it meant the end of collective bargaining.
But Mr Altman claimed salaries had actually "sky-rocketed" as ambitious charter schools competed for the best staff. "Teachers are treated like royalty now," he said.
Free to experiment
There are 3,000 charter schools in the US, some run for profit by companies, others set up by universities or activists who think they can do better than the traditional public school.
They are publicly funded but released from certain rules and regulations that apply to other public schools on condition they produce better results.