Gates thought he was exempt from the requirement

The deal involved Mr Gates's own investment firm and was not related to Microsoft, which continues to face anti-trust litigation.

Mr Gates failed to inform officials of his acquisition of more than $50m in shares in drugs company Icos Corp.

Mr Gates, the world's richest man, is estimated to be worth more than $40bn.

By US law, certain individuals and firms have to notify regulators before they can complete acquisitions of stock or assets worth more than this amount.

Mr Gates reportedly thought he was exempt from the rule because the purchase was for investment purposes.