Private companies with financial links to Tory politicians have won NHS contracts worth £1.5bn in the past two years, according to research by the UK’s largest trade union.

Unite claims that 24 Conservative MPs and peers who voted in favour of the government’s health reforms have links to 15 private companies that have won up the contracts since 2012.

Unite general secretary Len McCluskey said: “David Cameron promised there would be no top-down reorganisation of the NHS but he lied. How can we be in a situation where dozens of his MPs, voted for the sell off act and had links to private healthcare companies, knowing this would open up new opportunities for the companies that pay them.”

The Conservatives strongly denied any wrongdoing. “Any suggestion of impropriety is malicious and defamatory and will be treated as such,” said a spokesman. “All donations to the Conservative party are fully permissible and are declared with the Electoral Commission in accordance with the rules.”

Unite’s study comes as the main parties battle to gain the public’s trust over the future of the NHS before next year’s elections.

At the Conservative party conference earlier this week Cameron pledged to protect the NHS budget in real terms from 2015 to 2020.. The week before Labour leader Ed Milliband unveiled plans for a £2.5bn Time to Care fund to pay for 3,000 midwives, 20,000 nurses, 8,000 GPs and 5,000 care workers.

Unite’s research includes politicians it says have received donations from organisations linked to private healthcare companies. It also includes others it says have a financial stake in companies that have won contracts since the 2012 Health and Social Care Act.

McCluskey said around £12bn of former NHS services are now being run by the private sector.

“Key clinical services including cancer care, blood analysis and mental health have been sold off or are up for sale,” said McCluskey. “It is time to scrap the Health and Social Care Act and save our NHS.”

Among those highlighted in the Unite research is former health secretary Andrew Lansley, the chief architect of the 2012 act. Lansley received a donation of £21,000 from Caroline Nash, the wife of John Nash, in 2009. At the time John Nash was chairman of Care UK, one of the UK’s largest health companies, which, according to Unite, has won more than £650m in NHS contracts in the past two years.

A spokeswoman for Lansley said he “neither solicited nor directly received any money from John Nash”. She said the money came through Tory head office and was then allocated to Lansley’s private office. “Andrew himself would not have known the source of the donation, but for the proper and full disclosure which was made at the time of the donation.”

A spokesperson for Care UK said the donation had been made in a personal capacity, adding Nash had stepped down as company chairman in April 2010.

“Unlike the trade unions, Care UK does not donate funds to any political party and never has,” she added.

Unite’s research also highlights the case of Lord Popat, who until 2013 owned the TLC group, which provides care home services. Unite said the group was now run by Popat’s wife and has won contracts worth £4.43m since the HSCA was passed in 2012.

Popat, who was made a Tory peer in 2010, denied any wrongdoing saying he was no longer involved in the “work, management or oversight of the company”.

“I absolutely refute any suggestion that I have acted inappropriately,” he said in a statement. “I am always extremely conscious of the need to avoid conflicts of interest, and that is why I have separated myself from any business interests I had.”

He said the HSCA had not changed the commissioning process for social care, adding: “It is still undertaken by local authorities – not the NHS - as it was before 2010. The notion that I have benefited from my supporting the legislation is therefore wrong.”

Unite said the research spanned a period of around a year and was based on freedom of information requests, analysis of company reports and clinical commissioning group accounts, as well as the electoral register and register of interests.

A spokesman said it had focused on Conservative politicians as they had backed the government’s health reforms. He said the union was undertaking research into links between Liberal Democrat MPs and peers and private companies but that was not finalised. He said it was not conducting a similar analysis of Labour figures as the party opposed the coalition’s health reforms.