Giving a key speech last September at a community college in south London on the future of free schools, Nick Clegg, the deputy prime minister, was clear. "Let me reassure you: yes to greater diversity; yes to more choice for parents. But no to running schools for profit, not in our state-funded education sector."

The announcement was trumpeted as a Liberal Democrat victory over the Tory "obsession" with market mechanisms. This weekend, that reassurance looks a little hollow.

Last week the education secretary, Michael Gove, gave the green light to Breckland Middle School in Suffolk to be renamed IES Breckland and run under a £21m, 10-year contract by Swedish for-profit firm Internationella Engelska Skolan (IES). The introduction of a profit-seeking company into the management of the school is allowed because of a technicality: the founder of the school is a charitable trust that has decided to outsource the entirety of the management to a fee-charging company – whose global business has a turnover of £60m a year, earning profits of £5m, according to analysis by the Adam Smith Institute.

The development is set to open the floodgates. Today the Observer can reveal that for-profit firms, encouraged by what is happening at Breckland, now plan to run more schools in what promises to be a watershed in British education. The Observer has learned that:

■ Two Swedish companies, IES and Kunskapsskolan – a similarly sized Swedish firm that already runs three academies on a not-for-profit basis – now aspire to manage chains of between five and 10 free schools on a fee-earning basis to create economies of scale.

■ Wey Education, one of the unsuccessful bidders for the Breckland contract, told the stock exchange in December that a market opportunity brought about by "the deconstruction of the education function within local authorities" offers a clear potential to "make a substantial return to investors and improve education in the UK".

■ The same firm, run by Zenna Atkins, the former chair of Ofsted, hopes to make an "impact in a positive way" on the lives of 250,000 children over the next five years, while Wey's broker forecasts a turnover of £17.5m by 2014 and a £9.9m "bottom line", through providing services in the UK and abroad.

■ A shares prospectus for that firm spells out that "current teaching methods, allocation of resources, wastage and inefficiencies create [an] opportunity" to deliver education at a lower cost and provide a financial return.

A senior Lib Dem source has admitted to the Observer: "We didn't foresee this." But while it is clear that profit-making companies are now set to play a key role in the UK's education system – via the back door, critics claim – the question is whether that is a bad thing.

It is undoubtedly true that the private sector has long been a part of the fabric of the state school system. Under Labour, the rationale – largely borrowed from America and its philanthropic culture – was that private money could revitalise a cash-starved system. The private sector invested in schools, took over the provision of local authority services and built and managed buildings through the controversial private finance initiative. The charitable sector was even allowed to manage state-funded academy schools, a limited programme at that stage designed to help failing schools or those in underprivileged areas to raise their standards.

In a handful of cases – for example, Turin Grove school in Edmonton, north London, and the Priory school in Taunton, Somerset, a school for pupils with special educational needs – profit-making companies Edison Learning and Lilac Sky Schools were given short-term £1m contracts and permission to take a profit if they managed to lift the failing institutions. The results were impressive and the schools improved.

However, this time there is a difference in scale thanks to the extent of Gove's reforms: 45% of all state maintained secondary schools are now academies or about to convert, and there are now 1,529 academies in England, compared with 200 when the coalition came to power. In these changed circumstances, the involvement of the for-profit sector – taking advantage of the breaking down of local authority control, supervision and services – is set to explode.

And despite Clegg's rhetoric, there is ample evidence that Gove is supporting such a revolution, not least by establishing a new government "framework" of companies pre-authorised to offer project management and educational services for a fee.

James Grew from Policy Exchange, the thinktank at which Gove was formerly chairman, says he will publish research next month that challenges the opposition to profit-making in the schools sector, citing efficiencies and results enjoyed abroad.

Those involved certainly insist they have philanthropic aims and that the money they may make is an irrelevance when measured against the benefits they hope to bring.

Atkins, who earns £100,000 a year in her role at Wey Education, says she is working with seven potential founders of free schools and hopes to help them to manage their establishments once they are set up – at least in part because of the daunting nature of the task. Parents and governors, she says, having created an academy, "may realise they don't have the capacity and they don't have the risk appetite, because you have to have a big one to take everything on their shoulders and they want to contract that out – that's my business".

She is aware of the resentment in some quarters against the for-profit sector's involvement in schools, but believes that the issue of money-making is a "red herring" because any profit is only taken when efficiencies are made. The only judgment, she insists, should be whether the model works.

"Profit becomes a real issue if you control price," she said. "If you don't control price, as you don't in this instance, profit is irrelevant because the price is fixed.

"The only thing that is relevant is quality. You need to judge schools on how they operate, not on whether the operator is making 5% profit, because you don't care.

"If that 5% profit is making a far better school than one that is not making a profit or is making a bloody loss, you are interested in quality."

She added: "There are real risks with the private sector getting involved in state school education and there are real opportunities. And I think the trick is backing the right private sector organisation."

Steve Bolingbroke, managing director of Kunskapsskolan, added: "I have a problem with the phrase 'for-profit' that is used. I don't think we or anyone else in the market is interested in slicing 10% of the cost of a school. That is just a cost cut. We are interested in investing in schools and if we get good results and get lots of people to come to them then we might leverage a return on our investment.

"And the way to make that return is to make sure the schools are full, popular and run a number of them to ensure you make efficiencies across the schools."

Jodie King, the UK manager at IES, says her company will be scrupulously fair in assessing the fees it will charge for Breckland, and that IES is in talks with a further two free school groups over running their institutions for fees – but the firm has greater ambitions.

"It would be nice to go to one trust who, if they had 10 schools across the UK, we could go through procurement to have those 10 schools," she said. "It is what we are exploring."

But, while being insistent that her company is in the sector for the right reasons, she hints at the dangers of the new model, which sets up autonomous schools run by for-profit companies competing for pupils through results.

"It is awful, but we kind of have to accept failure more than we do at the moment. So if a school does fail because of its results, then that is right that it should fail – it should not be kept going at all costs. Yes, it is awful at that time for that year group, but surely the next year will be better for them rather than saying we are going to forsake the next five years of that child's education. So if there is an awful company out there, then they should be allowed to fail and then someone else can take over."

It is the consequences of failure on a child's education that concern critics of the for-profit sector. Christine Keates from the teachers' union NASUWT says she is so suspicious of the government's agenda that she believes detailed figures released last week on the revenues of state schools in England were merely designed to tempt private companies looking for investments.

She said: "They are publishing financial information about schools which is supposed to give parents choice, but actually all of this is about getting the public sector, and education in particular, in a position where it is an attractive option to private companies in terms of taking over and running schools, or in terms of providing services.

"Our concern is that a lot of the private companies coming in, particularly now the secretary of state has said they can be profit-making, are completely changing the ethos of why people get involved in education. If you are in the private sector, you are looking for a contract that is going to maximise your profits. When it is no longer financially lucrative, who picks up the fallout from that?"

The Department for Education disputes Keates's claims over the statistics. Of Breckland School, a spokesman said that "the free school's charitable trust has decided that it wants to draw on the expertise of an established education company, with a proven track record of running good schools. This is not the same as the free-school proposers making a profit themselves."

He added: "The charitable trust will manage the contract and hold the contractor to account, and will be fully responsible and in control over what happens in the school."