Cameron’s free school pledge comes under fire from Lib Dem Minister

However the plans were immediately criticised from within his own cabinet as Liberal Democrat Minister David Laws said it would “blow a giant £4bn hole in the school buildings budget” and risk a places shortage.

The proposals to open at least 500 new state schools, will mean an extra 270,000 places at the institutions which were introduced as part of former education secretary Michael Gove’s reforms.

Mr Cameron’s plans for a major extension of the policy if he remains in Number 10 came as the Government announced that 49 more free schools have been given the green light in the final wave of approvals before the election.

This includes New College Doncaster, a plan for a sixth form sister school of the existing New College Pontefract.

In a speech at a school in west London, the Prime Minister hailed the free school initiative as “the most successful schools programme in recent British history”.

Mr Cameron said: “If you vote Conservative, you will see the continuation of the free schools programme at the rate you’ve seen in the last three years.

“That means, over the next Parliament, we hope to open at least 500 new free schools, resulting in 270,000 new school places.

“Remember - we’re the only party that is committed to this. The only party that’s opening up the education system so we can get more good places for your children.

“And isn’t that what every parent wants - a great education for their child? You deserve the security of knowing your child is getting just that. And with the Conservatives you should expect nothing less.”

The announcement that 49 more schools have been given the go-ahead means more than 400 have already been approved under the coalition Government.

Labour has criticised the free schools policy, claiming it can lead to money being channelled to areas where there is a surplus of school places instead of being targeted at areas where classes are overcrowded.

Mr Cameron’s plans for an expansion of free schools came as a report by the centre-right think tank Policy Exchange suggested they were raising standards for all pupils in areas where they open, even for children who do not attend them.

The report suggested that the competitive effect created by a free school leads to improved academic standards in nearby under-performing schools.

However it also says: “It should be obvious - but bears setting out explicitly - that such data cannot demonstrate conclusively that any changes seen are as a response to the new free school.”

The report was also criticised by the Local Schools Network campaign group.

Henry Stewart said: “The argument for free schools is that they introduce competition that will improve the performance of all schools in the area. This is the basis of Cameron’s argument and is the argument tested in the Policy Exchange document. However the data clearly does not support this. Overall the change in results of schools closest to a free school are remarkably similar to those nationally.”

Education Secretary Nicky Morgan said the free schools programme was a “huge success story” and would “continue to expand”.

“We know from our own evidence that 72 per cent of free school heads say that they are having a positive impact on local schools in their area, they are driving up standards and they are giving parents more choice about having a great local school in their area,” she told BBC Radio 4’s Today programme.

“These are schools where the money comes directly from the department to the schools, where it is run by those who know the children best, who know education best. I think that is very important.”

Ms Morgan played down suggestions that many schools were not full as they were being built in areas that already had enough places for pupils.

Some 90 per cent of the schools approved were in locations where there was a lack of provision, she said.

She also insisted she could rule out a Tory government allowing free schools to make a profit.

“Yes, I can. I think having profit in education is something that would make me feel very uncomfortable,” she said.

A Liberal Democrat spokesman said: “This proposal simply masks the deep cuts the Tories are planning for education. Their scorched-earth policy for public services would mean that funding for nurseries, schools and colleges could be slashed by a quarter by 2020.

“In government, the Liberal Democrats ensured free schools were only set up where there was a genuine need for places, we blocked Tory plans to introduce profit-making in schools, and we stopped Michael Gove’s plans to cut the schools budget in 2010.

“A majority Tory government is the biggest threat to our schools and public services. The Liberal Democrats have put education on the front page of our manifesto, committing to protecting the education budget from cradle to college. It is clear that the Liberal Democrats are the only party committed to protecting education.”

The general secretary of the Association of Teachers and Lecturers (ATL), Mary Bousted, said Mr Cameron’s announcement was “worrying for parents and taxpayers” and that Policy Exchange’s evidence for the benefits of free schools were “paper thin”.

“The report’s conclusion that ‘free schools are raising standards for other pupils across the local community’ is demolished by its own evidence that standards fall in the majority of established schools when free schools open in their local area,” said Dr Bousted.

“Nicky Morgan claims that 72 per cent of free school heads tell her their schools raise standards in their area, but what else would she expect them to say? She should despair that a Conservative flagship education policy is built on such weak evidence.

“But there are more fundamental problems with the Conservatives’ free schools policy. Parents will be concerned that the free school programme will not provide sufficient school places for the 18 per cent increase in the number of primary pupils in the next eight years.

“Taxpayers will be concerned that the DfE has had its accounts ‘qualified’ by the National Audit Office because it is unable to adequately account for academy and free school finances.”