Hundreds of thousands of people have signed a petition calling for Transport for London to reverse its decision not to renew Uber's licence.
On Friday, the regulator said Uber was not "fit and proper" to hold a London private hire operator licence on the grounds of "public safety and security implications".
More than 0.40 million (400,000) names have since been added to Uber's petition on Change.org.
The ride-hailing app firm says it will appeal against TfL's decision.
The petition, started by Uber London, says: "If this decision stands, it will put more than 40,000 licensed drivers out of work and deprive millions of Londoners of a convenient and affordable form of transport."
TfL's concerns include Uber's approach to carrying out background checks on drivers and reporting serious criminal offences.
Uber's licence is due to expire on 30 September.
It has 21 days to appeal against TfL's decision and can continue to operate while any appeals are ongoing.
Petitioner Glenn Gathercole, from London, said he added his signature because: "Uber provides a much needed alternative to minicabs and black cabs. It is more efficient, safer and economical than the alternatives."
But Mayor of London Sadiq Khan supports TfL's decision.
"It would be wrong if TfL continued to license Uber if there is any way that this could pose a threat to Londoners' safety and security," he said.
However, Uber said the move "would show the world that, far from being open, London is closed to innovative companies".
One Uber driver in London said: "I don't think it is a fair decision. Uber offers a flexible schedule, and a weekly income."
And Twitter user @Gabbysalaza_ said that she was "annoyed" at the decision as Uber allowed her to get out of "uncomfy" situations if out at night.
Some 3.5 million passengers and 40,000 drivers use the Uber app in London.
