The head of mental health charity Mind has resigned from a review of anti-depressants accusing a government drugs watchdog of negligence.
The regulator has warned doctors to prescribe just 20mg for most people
Richard Brook acted after GPs were advised to limit doses of the anti-depressant Seroxat.
He claims the Medicines and Healthcare products Regulatory Agency (MHRA) knew about the concerns for 10 years and he was pressured not to reveal them.
Mr Brook has left the MHRA's working group. The MHRA denies any cover-up.
Health charity SANE fears the controversy may deter thousands from taking essential medication.
MHRA is looking into the class of drugs called SSRIs (Selective Serotonin Reuptake Inhibitors) of which Seroxat is one.
Mr Brook accused the MHRA - which licenses drugs for use in the UK - of failing in its duty to the public by not acting on data showing thousands of people were taking unsafe doses of Seroxat.
He said the information only came to light last Thursday when the Committee on Safety of Medicines, which advises the MHRA, warned doctors to keep to 20mg when prescribing the drug to new patients with most types of disorder.
The committee revealed that 17,000 people were given higher doses last year, which could increase the risk of side effects such as insomnia and nausea.
There have also been claims Seroxat is addictive and can increase the risk of violent behaviour in some people.
Mr Brook said: "On Thursday the agency at last published information advising that many thousands of men and women in this country may have been taking Seroxat at a dose that was unsafe.
"What they failed to mention - and what I am now making public - is the fact that the regulator had the data on which the basis of this decision was made for well over a decade as part of the original licence application.
"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 well-being of the British public."
He called for a full and independent review of UK drugs regulation.
The Department of Health said in a statement: "The MHRA continuously monitors the safety of SSRIs, including Seroxat, and advises healthcare professionals and the public accordingly.
"The MHRA is aware of Mr Brook's concerns but denies any cover-up."
A spokeswoman said the MHRA had changed its advice whenever any new information came to light.
Dosage guidelines for those under 18 were revised last year, for instance, she said.
Patients on anti-depressants including Seroxat are warned not to stop taking their medication without a doctor's advice.
Mr Brook stressed that anyone with concerns about their medication should contact a doctor before taking any action, and should "on no account" simply stop taking an anti-depressant such as Seroxat without medical advice.
Mental health charity SANE are also anxious that those on medication do not stop treatment because of the negative media reports.
Chief Executive Marjorie Wallace said the controversy was "a pity":
"Our concern is that if the public loses faith in the safety of the newer medications many thousands will go untreated and others will be afraid to seek help. "
She added: "Seventy per cent of those affected by depression respond well to anti-depressants.
"We are worried that these reports will result in more people rejecting treatment, and further suicides."