Prosecutors have raided the Seoul office of Roche on suspicions the Swiss drugs group illegally provided Tamiflu to HSBC and Novartis amid fears of a widespread outbreak of swine flu.

The South Korean Food and Drug Administration confirmed that prosecutors searched Roche’s office on Wednesday as part of an investigation into large-scale illegal purchases of the medicine to treat influenza.

“HSBC and Novartis bought a lot of Tamiflu to give to their employees in case of emergency because it would be difficult to secure the medicine if swine flu starts to spread rapidly,” said Kim Young-kyun, an official at the regulatory agency.

HSBC declined to comment and Novartis did not respond.

Mr Kim said the two companies provided a list of staff that was used to obtain prescriptions for Tamiflu. The agency said it was illegal to prescribe medicine in South Korea without a patient undergoing a medical examination.

Prosecutors are still investigating the case and no charges have been filed.

“We had a lot of inquiries from companies about Tamiflu. We explained that anyone who wants it needs a prescription. At no time have we deviated from that position,” said Roche.

The South Korean authorities descended on the Swiss company’s office in Seoul after obtaining sales data for Tamiflu from the regulatory agency. “We got the purchase data and prosecutors raided the Korean office of Roche to see whether any other companies were involved and [to] determine exactly what role Roche played in the process,” said Mr Kim.

The investigation comes amid mounting concerns in South Korea about a potential widescale outbreak of swine flu.

Seoul this week raised the swine flu alert to “red”, the highest level in a four-step warning system, after the number of newly infected patients doubled last week.

With the peak flu season expected in coming weeks, South Korean health health officials hope to have sufficient supplies of Tamiflu and other antiviral drugs for 11m people, or 20 per cent of its population, by the end of the year.

Tamiflu has become one of the world’s best-selling drugs thanks to growing fears of a global flu pandemic. Sales are projected to reach SFr2bn ($1.96bn) this year.

Additional reporting by Haig Simonian in Zurich