The NHS is paying private hospitals £35m a month to treat patients - a rise of more than 60% in a year, figures obtained by the BBC show.
The October 2010 data showed the NHS paid out the money for nearly 17,000 patients to have non-emergency care.
Doctors have criticised the trend, saying it is a luxury the NHS cannot afford in the current economic climate.
Under England's patient choice scheme, people can choose a private hospital if treatment can be done at NHS prices.
In October 2009, 12,283 patients took up this option at a cost of £21.6m. By October 2010, that figure had risen to nearly 17,000 at a cost of £34.7m.
The rise has happened during a period when the number of private hospitals willing to take NHS patients has hardly changed.
Instead, doctors have said it has been driven by an increase in marketing by private firms.
GPs have reported being bombarded with leaflets from leading private health firms, while adverts have been taken out in newspapers promoting the ability of NHS patients to choose private hospitals for a range of treatments, including hip and knee replacements, eye surgery, the removal of skin lesions and hernia operations.
And despite the numbers opting for private hospitals only representing a small fraction of the overall numbers under-going elective treatment, the British Medical Association has questioned the wisdom of the scheme as the majority of hospital income is linked to how many patients they see.
The initiative was launched four years ago at a time when the NHS was getting bigger rises in its budget and was striving to get waiting lists down.
But it is now being questioned amid concerns about how the health service will cope in coming years.
The House of Commons Health Committee this week warned the NHS would be "tested to the limit" because of the savings it has to make.
Paul Flynn, of the BMA's consultants committee, said: "We have to wonder whether this is an expensive luxury for the NHS at the moment.
"It is facing a tough time financially and it is perhaps the right time to rethink this.
"The risk is that it will destabilise NHS trusts. It is growing and you can see why private providers are interested at a time when things like private healthcare insurance is feeling the pressure of the downturn."
Ali Parsa, head of Circle, a health firm which is part owned by its staff and registered for patient choice, agreed the NHS market was becoming more important.
He said about a third of the patients Circle sees at its flagship hospital in Bath were NHS patients, but this could tip the 50% barrier in the future.
"We will be doing more marketing in the future. We want to show what can be done. NHS patients get virtually the same treatment as our private ones and they are amazed by what they get.
"I think this is good for patients and good for the NHS - it relieves some of the pressure and creates competition."
A Department of Health spokesman said choice was "fundamental to a truly patient-centred" health service.
He added: "Choice doesn't just benefit those patients who choose to go to a particular hospital.
"It gives hospitals a strong incentive to tailor services to the specific needs and preferences of all patients which, in turn, should lead to better outcomes and the reduction of health inequalities."