A fifth (20%) of the adult population has spent around £20.7 billion on private operations and medical procedures (excluding cosmetic surgery) over the past 10 years - the equivalent of around £2.07 billion a year.
Private patient finance firm Pay4Later's poll of 1,031 UK adults found the average one-off cost of procedures, which was not funded by private medical insurance, was £2,185.
Around 3.31 million spent up to £499 on private treatment over the past 10 years.
Meanwhile, the research showed that around 321,538 people spent £10,000 or more on treatment.
Scott Law, CEO of Pay4Later said: "The NHS does a brilliant job but everyone knows that with an ageing and growing population, it is coming under greater pressure.
"Our research shows that rather than wait for their medical procedure on the NHS, many people are deciding to pay for it privately. As the pressure mounts on the NHS, we expect this trend to continue."