Shares in Irish biotech group Elan slipped yesterday as US drug regulators delayed the expected return of its breakthrough multiple sclerosis drug Tysabri to market.

The Food and Drug Administration informed Elan and its US partner, Biogen Idec that it would need up to 90 more days to consider a revised risk management plan put forward by the companies for the drug, which was involved in the death of two clinical trials patients.

Shares in the firm dropped 8 per cent in Irish trading hours to close at €11.20, having dipped to €10.20 at one point. In New York, the shares opened more than 5 per cent lower before rallying slightly. They ended the US session 3.83 per cent weaker on $13.80 .

A spokeswoman said the news was disappointing, both for patients and for shareholders.

"The two companies had been working on the basis of a decision being made by March 29th but there was always a chance that this might be delayed.

While it is unusual for the FDA to exercise such extensions, it is not unprecedented. The companies noted that the FDA announcement was an issue of timing, rather than of whether the MS drug - seen as considerably more effective than existing MS treatments - would return to market.

While the new deadline gives the FDA up to June 29th to rule on the MS drug's risk management programme, the outcome may be known sooner.

The European approval process for the drug remains on track, according to the companies. They expect a decision by the summer which would allow Tysabri to go on sale here in the second half of the year.

The FDA delay does not affect clinical trials of the drug whose resumption had previously been sanctioned.