"We are highlighting a scam," said Claire Donaldson, Society vice-president, who proposed an SoR motion at the recent TUC Congress in Brighton.
Originally passed at the Annual Delegates Conference (ADC), the motion called on the TUC to campaign against money being offered to private patients to switch their treatment to the NHS.
With tough NHS waiting targets, particularly for cancer care, the Society claimed that private insurers are persuading patients to use NHS services.
The SoR motion said it is "scandalous and unethical for private health care insurance companies to offer a financial incentive to patients who opt to continue with NHS funded care to save the insurers money.
"This reimbursement is a fraction of the total cost of treatment and thus saves the insurance companies vast sums of money which instead is delivered to shareholders."
Claire said, "Our motion is not an attack on an individual's right to choose private healthcare (but) these companies take money from their clients, offering the promise of quicker, better healthcare.
"In many cases, the NHS provides not only an equal or better level of care, but often also the staff who provide the care for these patients," she continued.
"The insurance companies cannot cost effectively provide care and treatment to cancer patients, who need a lot of expensive, specialised equipment. A patient who has paid for private health insurance is offered a financial incentive to access treatment and care through the NHS. It is outrageous that private companies can financially benefit."
The motion passed unanimously.