Up to 5,000 mental health patients may have been sectioned over the last 10 years by doctors who had not undergone proper approval, Health Secretary Jeremy Hunt has said.
Mr Hunt said there were "irregularities" around the way doctors were approved before they could assess patients for detention under the 1983 Mental Health Act.
He told the Commons: "Our latest best estimate is that 2,000 doctors were not properly approved and that they have participated in the detention of between 4,000 and 5,000 current patients within institutions in both the NHS and independent sectors."
Mr Hunt said four of England's Strategic Health Authorities were affected, with some patients sent to Ashworth and Rampton secure hospitals, home to some of Britain's most notorious prisoners.
The Health Secretary said emergency legislation would be rushed through Parliament to deal with the scandal, which he learned of last week.
But he added: "There is no suggestion that hospitalisation or detention of any patient has been clinically inappropriate, nor that the doctors so approved are anything other than properly qualified to make such recommendations, nor that these doctors might have made incorrect diagnoses or decisions about the treatment patients need.
"All the proper clinical processes were gone through when these patients were detained. We believe no-one is in hospital who shouldn't be, and no patients have suffered because of this."
He said doctors recommending patients to be locked up would not have known they had not been properly approved to make such recommendations. "They acted in good faith," he added.
Shadow health secretary Andy Burnham said: "Detaining people under the Mental Health Act raises the most serious issues of fundamental rights and of patient and public safety. Any reported failure will therefore always be a matter of the highest concern."
Mr Burnham said MPs wanted to "get to the bottom of the unacceptable breaches of procedure we have just heard about".