The news comes as the World Health Authority (WHO) officially classified the outbreak as a pandemic, the first official flu pandemic in 40 years.

Legal Week reports that Allen & Overy ordered the stocks of the anti-viral drug Tamiflu - the primary means to treat the new strain of flu - several years ago as part of its business continuity planning as the drug has broad applications for viral infections.

Allen & Overy has enough of the drug to treat its workforce, across 31 major centres, and is holding the drugs locally across its global network through third-party medical providers, with the exception of jurisdictions where local regulations restrict such a policy.

A spokesperson for the firm said: "This provision forms part of our business continuity planning, which includes a number of different contingency plans and measures to ensure we are as prepared as we can be for all eventualities that could impact our operations.

"Since the issue of swine flu emerged we have kept our people informed of the latest advice and guidance from the WHO and other organisations and issued updates to them as and when appropriate as the situation developed globally."

The disease has so far been confirmed in more than 27,000 cases worldwide - including 822 cases in the UK, though the strain has so far proved less severe than initially feared.

Meanwhile, fellow law firm Lovells has revealed its Hong Kong office had been affected by the disease after an attendee to a local student day tested positive for the flu strain.

The firm's office was cleaned after local health authorities were contacted and staff members involved have been quarantined. One staff member has subsequently fallen ill with the virus.

A Lovells spokesperson commented: "In London we have a small working group that has looked at some of the implications of an outbreak and have also been working with our occupational health advisers to plan what we might do in the event of having a case, but first and foremost we follow the appropriate health authority guidelines.

"We also have substantial stocks of Tamiflu, but since it requires a doctor's prescription before it can be administered we cannot simply hand it out. We would not expect to be a primary dispenser of the drug since that is the role of the health authorities but we do have a supply as a reserve contingency."

A number of law firms are currently reviewing their policies to respond to the disease.

Clifford Chance has also formed a taskforce to stay on top of developments on the spread of the virus and has provided staff with guidelines on what to do should they believe they have been affected.