A warning by the regulator who spearheaded investigations into Vioxx that five further drugs may have safety concerns has prompted a plunge in shares in major pharmaceuticals companies.

AstraZeneca stock slumped by 10 per cent, wiping more than £3.5 billion off the company's stock market value this morning. The market capitalisation of GlaxoSmithKline, Europe's largest drugs maker, fell by £3 billion after it stock slid by more than 4 per cent.

The slides followed a claim by Dr David Graham, a regulator at the US Food and Drug Administration, of safety concerns surrounding AstraZeneca's Crestor anti-cholesterol drug and Serevent, an asthma treatment made by GSK.

"I can tell you right now there are at least five drugs on the market today that need to be looked at quite seriously to see if they belong there," Dr Graham told a committee of US senators.

But the FDA this afternoon moved to distance itself from Dr Graham's comments. "The five specific drugs that Dr Graham identified in his oral testimony are currently approved as safe and effective for use in the United States," Dr Steven Galson, the acting director for the FDA's Center for Drug Evaluation and Research, said in a statement on the FDA Web site.

"FDA takes all allegations of safety risks seriously and will continue its longstanding practice of assessing the safety of all drugs using only sound science and peer-reviewed analysis and literature."

The other drugs named by Dr Graham were Meridia, a weight-loss drug made by Abbott Laboratories, Bextra, a painkiller made by Pfizer, and Accutane, Roche's treatment for acne.

He said that the FDA, the US watchdog responsible for ensuring drug safety, had "case reports of people dying clutching their Serevent inhalers.

Dr Graham added that the FDA, as currently configured, was "incapable of protecting Americans against another Vioxx".

"It is important that the American people understand that what happened with Vioxx can happen again," he said. "We are virtually defenceless."

Merck withdew Vioxx in September after evidence linked the arthritis pill to heart problems. Merck stock fell by more than one-quarter to an eight-year low when the withdrawal was announced and has continued to suffer amid reports that the US pharmaceuticals company ignored concerns over the drug lodged over four years.

However, the FSA has distanced itself from Dr Graham's comments. "I do not have any reason to believe that set of five drugs is specifically more concerning," Dr Sandra Kweder, deputy director of the Office of New Drugs, said.

An AstraZeneca spokesman said: "To date, the FDA has not given us any indication of a major concern regarding Crestor".

GSK said that it stood firmly behind Serevent, "which is safe and effective when used appropriately and in accordance with labeling and treatment guidelines".

"Any issues concerning mortality associated with Serevent have been fully considered by the FDA," the company added.

Seymour Pierce analysts remained unmoved by Dr Graham's comments, adding that he was known as a "bit of a loose cannon".

Merrill Lynch noted that Serevent was of limited importance in GSK sales, although Crestor is set to account for 12 per cent of AstraZeneca takings in 2008.

AstraZeneca shares stood 241p lower at 2126p in afternoon trade after falling below 2190p earlier. GSK stock stood 49p lower at 1143p.

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