Potential organ donors have been warned to be wary of organ donor register sites which are offering "financial arrangements" regarding donation.
The Human Tissue Authority (HTA) has issued a warning for people considering registering with services other than the NHS's Organ Donor Register.
It said that it has become aware of an organisation, Organ Tree Ltd, which encourages "recipients and donor families to reach financial arrangements".
Organ Tree encourages people to sign up to their register as a donor or recipient on its website.
Potential donors are told that their nominated beneficiary will get an "agreed amount to help offset funeral costs resulting from the donor's death" from the person who would receive their donated organ.
Meanwhile, recipients are offered "the option of accepting the donated organ and reimbursing the donor's beneficiary an agreed amount to help offset funeral costs resulting from the donor's death", after registering with the site for a 30 euro fee.
But the HTA, which regulates the donation of organs from living people across the UK, said the law prohibits almost all payments associated with organ donation.
A new position statement from the regulator states: "If an individual were to register with Organ Tree Ltd, or any other service operating on a similar basis, or enter into any financial arrangement associated with organ donation, they are likely to be breaking the law, which prohibits almost all payment and attempts to seek payment."
It said that anyone considering this service, or similar services, to be cautious and if in doubt, seek legal advice.
The HTA said it was highlighting risks associated with other services, and to help ensure that donated organs are used safely, ethically, and with the proper consent.
HTA chief executive Allan Marriott-Smith, said: "As the regulator, the HTA's role is to make sure donors give appropriate consent for the donation of organs for transplantation. Consent must be freely given.
"We understand the anxiety of those waiting for an organ transplant in the UK. We are constantly impressed by the incredible generosity of those willing to donate organs to save the life of another person.
"Signing up to the NHS Organ Donor Register is how you can record your decision about donating organs and tissue after death.
"Anyone in the UK signing up to an alternative service, where payment linked to donation is a possibility, is at risk of committing a criminal offence.
"You can be prosecuted for offering or receiving a reward in return for a donated organ, whether or not a transplant goes ahead.
"If members of the public have concerns about alternative organ donation websites, please contact the HTA.
"We are also happy to answer any other questions about organ donation or the use of human tissue in general."
Sally Johnson, director of organ donation and transplantation at NHS Blood and Transplant, added: "NHS Blood and Transplant has been made aware of a web based business, Organ Tree Ltd, offering payment for deceased organ donation.
"We have absolutely no association with this business.
"NHS Blood and Transplant is the organ donation organisation for the UK and we are responsible for matching and allocating donated organs.
"We wish to make it clear that deceased organ donation in the UK is an altruistic act.
"Giving or receiving payment or reward for organ donation is illegal."
:: For more information about the Organ Donor Register visit: www.organdonation.nhs.uk