The shadow health secretary Andrew Lansley's personal office received a £21,000 donation from the chairman of private healthcare firm Care UK, the Daily Telegraph has reported.
Care UK is an independent provider of healthcare services including the running of GP surgeries and NHS walk-in centres.
The Telegraph has reported that John Nash, the chairman of Care UK, made the donation in November.
The Conservative party has already suggested an increase in the use of private providers if elected.
According to the Daily Telegraph report, Health Secretary Andy Burnham is planning to write to David Cameron, the leader of the Conservative party, asking him to guarantee that his party's health policy will not be dictated by private health companies.
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 Conservative central headquarters. Mr Lansley did not solicit this donation. Donations from private individuals in no way influence policymaking decisions."