Private healthcare groups face having to sell off sites after the competition watchdog found 101 private hospitals across the UK have little local competition.
The Competition Commission ruled that almost 20 hospitals may have to be sold because stifled competition is pushing up prices and medical insurance premiums for private patients.
Incentives to doctors could also be slashed, and private hospitals could be barred from further tie-ups with NHS hospitals in areas where there is little competition, the Commission concluded.
The regulator said some private hospitals are in clusters owned by three major groups, Spire, BMI and HCA, which it said have been earning high profits in recent years.
About 80% of private hospital patients are funded by insurance premiums, typically paid for by employers. Insurers include the likes of Bupa and AXA PPP.
Entry by new players into the private hospitals market is rare, the Commission said, due to high costs, the response from existing operators and flat demand.
It found the lack of competition in many local areas means insurers have little choice but to use the incumbent operator - meaning higher premiums for all patients. Patients who fund their own care are also hit with higher charges in areas with little competition.
The regulator found HCA charges significantly higher prices to insurers than other operators, with BMI the next most expensive for insurers.
The Commission said the three major players' dominance caused "consumer detriment" of £173 million to £193 million a year between 2009 and 2011.
Commission chairman Roger Witcomb said: "The lack of competition in the healthcare market at a local level means that most private patients are paying more than they should either for private medical insurance or for self-funded treatment.
"The lack of available and comparable information, often less than is available to NHS patients, also makes informed choices - which could help drive competition - for these patients difficult."
The Commission also found private hospitals offer access to resources and perks to consultants to encourage them to use their facilities and refer patients. The watchdog said this could create an incentive for excessive tests or consultations.
Hospital groups will be banned from offering incentives to consultants to refer patients to, or treat them at, their hospitals, the Commission ruled.
Operators will also be prevented from using their dominance in a local area as leverage in negotiations with insurers.
They will also have to give more information on quality, fees and services.
Damien Marmion, managing director of insurer Bupa Health Funding, said the Commission's provisional findings were good news for patients and private healthcare.
He said: "It could be the treatment required to drive better value, better quality and greater transparency for customers.
"Millions of people with health insurance rightly expect high-quality healthcare for an affordable price. By tackling the lack of competition that has damaged the sector for too long, the Commission has understood the need for strong action and has put patients first."
Stephen Collier, chief executive of BMI Healthcare, insisted its profits are reinvested back into its hospitals.
"We reject absolutely any assertion that BMI Healthcare and its hospitals exercise market power or that we make excess profits at the expense of patients," he said.
"The vast majority of BMI's 69 facilities, in a UK market with over 500 rival facilities, face very significant local competition from other private hospitals and, increasingly, from the NHS."
Two larger insurers, Bupa and AXA PPP, achieve significantly lower prices than smaller insurers and have some buyer power, the Commission said, but this is not enough to offset the market power of BMI, Spire and HCA.
It added that insurers, particularly Bupa, need to communicate better with patients about what their premiums entitle them to.