More patients will have to pay 'top-up' fees for private care because of budget cuts in the NHS and long waiting times, a group of doctors say in a report.
More patients are going private, say Doctors for Reform
Doctors for Reform says the idea that health care is free across the UK is now a "political mirage".
The group has written to Health Secretary Patricia Hewitt calling for a debate on NHS funding.
A Department of Health spokeswoman said patients had always had the choice of paying for private healthcare.
The report, published by Doctors for Reform, was written by three doctors, including Karol Sikora, professor of cancer medicine at Imperial College School of Medicine.
Mr Sikora said: "Having to 'top-up' NHS care is a reality for many patients.
"But the political debate continues to perpetuate the mirage of a service completely free at the point of delivery".
Doctors for Reform says it is an independent, non-party group supported by almost 900 doctors.
Its report says patients are developing sophisticated approaches to 'topping up' NHS care with private treatments, including in key areas like cancer and heart disease.
It blames patchy provision of NHS services across the UK, long waiting times and varied quality.
The report also pointed to the falling cost of private treatments due to advances in technology and increased competition between firms.
"Without reform to health funding, the use of 'top-up' payments is likely to increase due to the upwards pressure on medical costs, the limits to tax-financing and, most importantly, the increasing importance of consumer choice," the study said.
In the letter to Ms Hewitt, shadow health secretary Andrew Lansley and Liberal Democrat health spokesman Norman Lamb, Doctors for Reform sets out three key questions.
It asks whether the NHS can guarantee a universal service in future, whether doctors inform patients of 'top-up' options as part of their general duty of care and how can access to healthcare be made fairer.
Christoph Lees, a consultant in obstetrics and maternal-foetal medicine at Addenbrooke's Hospital in Cambridge, added that increasing use of the private sector may disenfranchise those unable or unwilling to pay for 'top up care'.
A Department of Health spokeswoman said: "We will never change the values of the NHS - universal, tax-funded, free at the point of need.
"Nobody should have to pay for any available NHS service, but patients have always had the choice of paying for private healthcare if they so wish."