Five family doctors have this week become millionaires from the sale of their NHS-funded firm to one of the country's biggest private healthcare companies in a deal that reveals how physicians can potentially profit from government policy in the new NHS.

On Tuesday, the private health company Care UK announced that it had paid £48m for England's biggest out-of-hours GP service, Harmoni, originally set up as a GP co-operative, creating a new private health concern that could treat 15 million patients. The deal shows how GPs could profit from the coalition's health reforms by using their expertise to bid for contracts, then cashing in when a corporation buys them out.

Labour expressed concerns that profits may siphon off cash meant for patients. The Care UK deal rests in part on the government's drive to privatise England's non-emergency telephone service. Harmoni had won 12 contracts to run the new 111 non-urgent phone line, beating off competition from NHS Direct, the current state-backed provider.

It emerged on Friday that NHS Direct is to close 24 of its 30 call centres with the loss of more than 750 jobs. The trade union Unison said large centres in Bristol, Sheffield, Wakefield, Nottingham, Hull, Stafford, Chelmsford and Newcastle were among those due to close.

In contrast, the biggest winners from ministers' push on 111 appear to be a private equity group, ECI – which can take about £20m from the Care UK purchase of Harmoni – and a group of GPs who will share a pot of more than £25m.

Harmoni's finances are complex, with hundreds of shareholders and different classes of stocks. According to an analysis of documents filed with Companies House, five GPs figure prominently and own a quarter of the company between them. If each share has an equal stake, the GP founders of the company, David Lloyd and Nizar Merali, would share £2.8m. This could easily more than double, if just their preference shares are valued, to £6.3m.

Another winner seems to be NHS reform champion Ian Goodman. The north-west London GP chairs the Hillingdon clinical commissioning group and was also a board director of Harmoni. He could make as much as £2.6m. During the passage of the bill, he told reporters that he had declared his interests and would "quite rightly be excluded from any decision-making process where Harmoni was bidding". Goodman did not return calls or emails last night.

Another two London GPs, Adrian Richardson and Thomas Davies, are in line for payouts of more than £1m each. Scores of GPs in north London, who were originally members of the medical co-operative, also have small stakes in the company, which appear to be valued at £35,000 each by the deal. Care UK and Harmoni said they "can't comment on any beneficiaries of the acquisition".

Even for well-paid doctors, this is big money. At present, fewer than 3% of GPs earn more than £200,000 a year. Labour has warned that GPs' conflicts of interests – in essence rewarding themselves for providing cut-price care – could be as big a scandal as MPs' expenses.

The shadow health minister Jamie Reed told the Guardian: "As the private sector takes over out of hours services, it's a dangerous mix of medicine and the money motive. Ministers must not let profits come before patients; vital services must not be pared down to the bone."

Harmoni was the largest private firm providing out-of-hours services for the NHS and funded by the taxpayer. Although it was set up in 1996, it began its rapid growth in 2004 when GPs were allowed to opt out of out-of-hours care. Harmoni proved successful in picking up contracts to provide the service, relying in part on its network of GPs. In five years its revenue has grown fourfold, from £23m to almost £100m in 2012.

The company's success in winning the 111 contracts has made it even more valuable. From April 2013, people seeking urgent healthcare advice will dial 111 – the service replacing the nurse-led NHS Direct helpline in England. Care UK recently lured a top civil servant from the Department of Health – Jim Easton, who oversaw the NHS 111 procurement process – to become its managing director.

The Royal College of Nursing said the government had let NHS Direct be "effectively abolished" to save money, and claimed the new service would lead to more ambulance call-outs and greater pressure on accident and emergency departments.

Reed warned that NHS Direct could be making more than half its staff redundant. He said: "Not only are ministers axing the jobs of 6,000 hospital and community nurses, but they've now put hundreds of jobs at the vital NHS Direct service at risk too. It is deeply concerning that the Department of Health have been unable to resolve this issue – leaving half the service's staff facing redundancy.

"Worried parents and other patients who use NHS Direct will be unsettled by the news that the NHS is haemorrhaging dedicated and experienced staff."

In a statement, Mike Parish, Care UK's chief executive, said the acquisition of Harmoni would increase the company's ability to "improve unscheduled care and help health commissioners make sure that patients get the right treatment at the right place without unnecessarily occupying hospital beds".

He said: "By putting the 111 channel and Harmoni's expertise in healthcare technology alongside the existing out of hours expertise in both organisations, and Care UK's ability to invest in future service development, we can give the NHS better-quality care at lower costs."

A Department of Health spokesperson said:"It is completely wrong to suggest there was a drive to privatise the NHS 111 service. Procurement processes should be fair and transparent and judged not on who is cheapest, but who is best able to meet the needs of patients.

"Competition within the health service is about driving improvements in healthcare and enabling patients to access the best possible services. In fact the Health and Social Care Act 2012 is the first piece of legislation that introduces specific safeguards around competition in the NHS.

"The Act prevents discrimination in favour of private health companies over the NHS. It also introduces rules and regulations that ensure that patients continue to have access to the services they need."