Left untreated, it may result in respiratory failure and death.

In the four cases reported to the Centers for Disease Control and Prevention (CDC), in Atlanta, the patients were found to have been injected with a highly concentrated, unlicensed preparation of botulinum toxin A.

It had been intended for laboratory research, and was not intended for human use.

But a doctor at one unnamed clinic gave the three patients - and himself - courses of four to six injections as facial cosmetic treatments.

All four eventually reported symptoms of progressive weakness and abnormalities in the nerves that control functions such as movement of the facial muscles and swallowing.

Two also experienced shortness of breath.

The CDC researchers say the patients may have received doses 2,857-times the estimated human lethal dose by injection.

Although the precise lethal dose is not known, researchers point to primate studies suggesting it is around 40 micrograms per kilogram of body weight.

Tests prior to treatment showed three out of the four had blood serum toxin levels equivalent to 21 to 43 times the estimated lethal dose. Data for the fourth patient was not available.

The researchers say the patients did not die because they received prompt hospital treatment.

The doctor involved in the case pleaded guilty to misbranding a drug and was sentenced to three years in prison.

'Negative attention'

Writing in the journal, the team, led by Daniel Chertow, said: "Physicians and patients must be aware of the hazards associated with illegitimate use of unlicensed botulinum toxin products.

"Only licensed products should be used clinically."

In the UK, an expert working group submitted a report to the government in January last year calling for powers to regulate the cosmetic treatment industry, including who should be able to give botox treatments.

But there has so far been no decision on whether or not these recommendations will be accepted.

Paul Stapleton, head of the Mapperly Park Clinic, part of the Independent Healthcare Advisory Service which has backed calls for tougher regulation, said: "Problems like this case reported in the US could occur in the UK.

"Without regulation, there is nothing to stop unqualified and untrained people giving botox treatments."

He added: "We have been working for a number of years to bring regulation to a sector that has run rampant, bringing both damage and risk to public health.