Boris Johnson has given the controversial minicab firm Uber seven days to reveal whether it is based in London or Amsterdam.
The ultimatum came just hours before a mass protest by London taxi drivers which threatens to gridlock the West End. Uber uses smartphone technology and GPS satellites to calculate fares.
The cabbies claim it breaks a law which says only traditional taxis can use a meter. Uber and its drivers are licensed by Transport for London but there's confusion over where the company is based.
Uber's terms and conditions refer to a private limited liability company established in the Netherlands. Unless the firm can prove it is based in London, it could lose its right to operate. Today's demonstration is set for a showdown between cabbies and the police after Scotland Yard imposed tough restrictions.
Thousands of drivers are threatening to bring Trafalgar Square and surrounding streets to a standstill.