TORONTO -- Many private clinics in Ontario are misleading patients and billing them for medically unnecessary services, public health advocates charged Tuesday.

A "significant" number of clinics contacted by the Ontario Health Coalition were charging patients extra fees on top of billing the Ontario Health Insurance Plan for necessary procedures, such as colonoscopies, the group said.

One clinic was charging patients $50 "administrative fees" for such things as a snack and patient records, according to OHC, which worked with six university students to conduct the research.

Such fees were more common among cataract clinics, which were charging patients hundreds and even thousands of dollars for medically unnecessary tests and procedures when they came in for needed surgery, said OHC's executive director Natalie Mehra.

Clinics are manipulating patients, who are confused about what they need and what they don't need, she said. Some patients they spoke to were concerned that if they refused to pay extra fees, that they would get substandard care.

"We were extremely disturbed at the types of information given to patients to try to 'up-sell' them on these medically unnecessary tests and procedures," Mehra said.

"In almost every case, the tests and procedures were made to sound like they actually made a significant medical difference -- that they were safer and better, that they had better medical outcomes."

The governing Liberals must crack down on such user charges which are more widespread than ever, she added.

Health Minister Deb Matthews said she'll look into any unethical practices to protect Ontario's single-tier public health-care system.

It's illegal to charge patients for services covered under OHIP, she said, and people are welcome to call the government if they have questions.

But Matthews insists the specialty clinics are not for-profit operations and are providing care that is currently provided in hospitals.

"It's easier for patients and it brings down wait times and we're assured of the highest possible quality," she said.

"Why would we say no to patients, you must go to hospital."

But Mehra points to the auditor general's 2012 annual report, which said most of the 825 independent health facilities in Ontario were owned and operated by for-profit companies. Only three per cent are non-profit organizations.

The report said the government estimates that about half are fully owned or controlled by physicians, many of whom are radiologists who interpret X-rays and ultrasounds, for example.

The report also found that the Ministry of Health paid the clinics about $408 million in 2010-11 in so-called "facility fees" for overhead costs such as rent, staff, supplies and equipment. It also pays physicians a standard fee for each service provided.