John Nash, the chairman of Care UK, gave £21,000 to fund Andrew Lansley’s personal office in November.
Mr Nash, a private equity tycoon, also manages several other businesses providing services to the NHS and stands to be one of the biggest beneficiaries of Conservative policies to increase the use of private health providers.
Andy Burnham, the Heath Secretary, is planning to write to David Cameron, the party leader, asking for him to guarantee that Conservative health policy is not dictated by private health care companies.
A senior Labour source said: "This raises serious questions about Andrew Lansley's judgement and it is difficult to see how he can continue on in his role as shadow health secretary when it would appear that a private health care company is helping to fund the development of Tory healthy policy.
"David Cameron claims that the Tories are now the party of the NHS - it makes his claim look more absurd than ever.
"How can the public trust Cameron on the NHS when his health secretary is hand in glove with a big beneficiary of Tory health policy?
"There is an apparent contradiction at the heart of Tory health policy and we urgently need to hear a detailed read out of what this money was donated in exchange for or the money should be refunded."
It is the first private donation registered with the Electoral Commission by Mr Lansley since 2001 and it is not clear why he accepted the money just months before a general election. Although he will not profit from the donation, it will help bolster his political profile.
Care UK provided services for 500,000 people last year, working with all ten Strategic Health Authorities and one in three Primary Care Trusts. It runs hospitals, NHS Walk-in Centres, GPs’ practices and care homes. It currently runs 59 residential care facilities with 3,400 beds.
In a recent interview, a senior director of the firm said that 96 per cent of Care UK’s business, which amounted to more than £400 million last year, came from the NHS.
Earlier this month, the Conservatives pledged to increase the use of private providers if elected.
In the draft manifesto published by the Conservatives, the party promised to “open up the NHS to include new independent and voluntary sector providers.”
In official company documents, Mr Nash, who is also a major shareholder in Care UK, praised reforms proposed by the Conservative party.
“We welcome recent policy statements by the opposition Conservative Party in the UK which have substantially strengthened their commitment to more open market reform to allow new providers of NHS services and for greater freedom for patients to choose their GP and hospital provider,” he said.
Mr Nash, 60, also founded the private equity fund Sovereign Capital in 1988, which owns several healthcare companies, as well as the independent schools group Alpha Plus.
Along with his wife, Caroline, Mr Nash sponsors a city academy in Pimlico, London through their charity Future.
Mr Nash previously worked at Advent, the private equity firm, and Lazard, the investment bank. He has been the chairman of the British Venture Capital Association.
Mrs Nash gave the Conservative party £60,000 last September and Mr Nash gave £6,000 to held fund David Davis’ unsuccessful leadership campaign against David Cameron in 2006.
The donation comes amid growing scrutiny of Conservative backers in the run-up to the election. Unlike most Labour ministers, senior Conservatives routinely accept money to help run their private offices from wealthy individuals or companies. Some Labour MPs receive such assistance from trade unions.
However, there have previously been concerns over potential conflicts of interest from the Conservatives’ funding arrangements.
Alan Duncan, the then Energy spokesman, was heavily criticised after it emerged that his private office was being funded by Ian Taylor, the chairman of Vitol, a firm of oil traders.
The private office of George Osborne, the shadow chancellor, is funded by the hedge-fund bosses Michael Hintze of CQS and Hugh Sloane of Sloane Robinson.
A spokesman for the Conservative party said: “We have been completely transparent about this donation. It has been properly registered with the parliamentary register as well as with the Electoral Commission and is therefore fully within the rules.
“John Nash and his wife have a wide range of interests, of which CARE UK is just one. This donation to support Mr Lansley’s office was made through CCHQ. Mr Lansley did not solicit this donation. Donations from private individuals in no way influence policy making decisions.”