London mayor Boris Johnson tonight urged cabbies to abandon a mass protest which threatens to gridlock the West End.
The Mayor's intervention came as police chiefs imposed tough curbs on the demonstration to try to limit the chaos.
The cab drivers are involved in a bitter dispute with Transport for London over so-called "taxi apps" used by minicam firms to calculate fares.
The drivers of traditional black cabs accuse TfL of failing to enforce laws which ban minicabs from having meters.
Scotland Yard predicted 10,000 cabs could take part in tomorrow's protest.
Senior officers claimed lives would be at risk if police cars, fire engines and ambulances were stuck in traffic jams.
The police appeared to be heading for a showdown with cabbies after restricting the protest to Whitehall between 2pm and 3pm.
Boris Johnson said: "Whilst I understand the concerns of our cabbies around the growth of new technology it cannot be for me as Mayor to pre-judge the courts. They will decide on this issue.
"I would urge black cab drivers thinking of protesting tomorrow in London to think again.
"Protest will achieve nothing, indeed it will cost London businesses, employers and employees millions in lost trade, time and productivity.”