Patient choice is being undermined by poor monitoring of the quality of care being provided, a study says.
Cataract surgery is one of the operations carried out by treatment centres
The St James' University Hospital team surveyed over 100 local health bodies on how they monitor cataract surgery.
The British Medical Journal report said more robust monitoring was needed as various sources were used to judge standards of care in England.
The government said measures were being taken to align inspection between private and NHS providers.
From the beginning of this year, patients in England have been given a choice of at least four providers for non-emergency treatment, including one from the independent sector where possible.
Over recent years the government has expanded the role of the private sector in providing non-emergency operations by signing deals for independent sector treatment centres, which provide a variety of minor surgery including cataract operations.
But doctors have long complained that the public are unable to compare the quality of care between different providers.
James Johnson, chairman of the British Medical Association, has called for an overhaul of the system of regulation.
"We hear all sorts of things about differences in quality of care, but the truth is we just don't know," he said.
The Patients Association has gone one step further, calling for performance data to be published for individual surgeons.
At the moment the Healthcare Commission watchdog inspects both NHS trusts and private treatment sectors, but judges them on different criteria.
Researchers found the methods used to monitor care varied from area to area in the survey of 125 primary care trusts.
A combination of patient feedback, GP data and official inspections were used to judge care, they said.
But they said it meant monitoring was ad-hoc, with patients more likely to be given information of waiting times and access.
Report author Dr Paul Brogden said: "The public expect that safe cataract care should be commissioned for them and robust methods of monitoring need to be agreed and implemented so that patients can make a truly informed choice.
"These decisions need to be made not only for cataract surgery but also for other elective procedures that will increasingly be made available to patients through the patient choice scheme."
A spokeswoman for the Department of Health said the government accepted there were variations in care data but said there were plans to "beef it up" - the Healthcare Commission is carrying out a consultation to align their inspection processes.
But she added quality of care was not the only criteria patients used to decide where to have treatment.