Richard Brook had a unique position as a lay member of the expert working group on the class of antidepressants which includes Seroxat and Prozac.

His resignation came in protest at what he considered a cover-up by the regulators, after months of pressure on him not to reveal the review's findings that Seroxat has for years been prescribed by doctors in an unsafe dose and that the regulators had the evidence in their possession for more than 10 years.

Mr Brook's resignation sheds a rare light on the workings of the secretive drug regulation agency and its advisers, and will heighten public concern over their relationships with the pharmaceutical manufacturers.

The expert group drew the attention of the government's advisory Committee on Safety of Medicines to the dosage issue in October. Mr Brook, who had been invited to represent patient interests because of concerns that the CSM is too close to the pharmaceutical industry, pressed for the findings on dosage to be put into the public domain.

Mr Brook warned the CSM and the Medicines and Healthcare Products Regulatory Agency, which licenses drugs on the advice of the CSM, last week that he felt he had no choice but to go public because of the risks to patients. He was warned in a letter last Monday from the MHRA that he could risk prosecution under the Medicines Act 1968, which protects the commercial confidentiality of information from drug trials.

But Lord Warner, the health minister, to whom Mr Brook had expressed his concerns, intervened, and on Thursday the CSM put out a "reminder" to all doctors that they should prescribe Seroxat only at the recommended dose, which is 20mg. Last year 17,000 people were put on a higher dose by their doctors, running the risk of increased side-effects, which some have alleged include agitation and thoughts of violence and suicide.

In his letter of resignation to Kent Woods, chief executive of the MHRA, Mr Brook said although he was pleased the information was now in the public domain he was unhappy that the announcement had stopped short of acknowledging that the crucial dosage data had been in the possession of the MHRA since 1990.

"The nature of the announcement fails to make public the fact that the clinical trial data you issued was available to the regulator for over a decade," he wrote.

"Despite four major regulatory reviews during this period and considerable consumer reporting and disquiet, the Committee on Safety of Medicines failed either to identify or communicate these key facts. As far as I am aware, the MHRA has not seen fit to acknowledge or address what in my view appears to be extreme negligence."

He regretted the fact that consumers would now be without a voice in the review, but wrote: "I believe my continuation on the expert working group without a frank and open admission of these important facts has become impossible. Consumers are entitled to full information both about medicines and how the regulator operates."

The warning letter to him on Monday had made it clear to him that he could not carry on, he said. Mr Brook said it was unclear why the MHRA had not acted earlier to ensure nobody was on too high a dose.

"Either they didn't understand the full implications of the available medical data at the time or, worse, that data was fully understood and they failed to act. Either way it amounts to extreme negligence and a clear dereliction of the MHRA's duty to safeguard the wellbeing of the British public."

Mind is calling for an independent review of the workings of drug regulation with patient representation at its heart. They were backed by Charles Medawar of the consumer group Social Audit.

"Richard Brook's brave resignation is enormously significant, and the loss of the one independent consumer figurehead now calls into question the whole credibility of the CSM's review," he said.

"It is alarming that Brook was put under such pressure, and typical of the regulators to try to gag him - and then produce a statement that is profoundly misleading by omission. "Brook holds a public position of considerable importance, and the Department of Health is the major sponsor of MIND.

"Brook's resignation pretty much mandates a formal parliamentary investigation of the UK medicines control system.