Transport for London (TfL), the capital's transport authority, has not renewed the licence of Indian taxi app Ola over public safety concerns.
The cab company has been operating in London since February.
TfL said the firm reported a number of failings including more than 1,000 trips made by unlicensed drivers.
Ola said it will appeal the decision and has 21 days to do so. It can operate in the meantime, according to the appeal rules.
The transport authority said Ola did not report the failings as soon as it knew about them.
"Through our investigations we discovered that flaws in Ola's operating model have led to the use of unlicensed drivers and vehicles in more than 1,000 passenger trips, which may have put passenger safety at risk," Helen Chapman, TfL's director of licensing, regulation and charging, said.
"If they do appeal, Ola can continue to operate and drivers can continue to undertake bookings on behalf of Ola. We will closely scrutinise the company to ensure passengers safety is not compromised."
The ride-hailing company began operating in Cardiff in 2018 and has since spread to other UK locations.
"We have been working with TfL during the review period and have sought to provide assurances and address the issues raised in an open and transparent manner," Marc Rozendal, Ola's UK Managing Director, said in a statement.
"Ola will take the opportunity to appeal this decision and in doing so, our riders and drivers can rest assured that we will continue to operate as normal, providing safe and reliable mobility for London."
Last week, major rival Uber secured its right to continue operating in London after a judge upheld its appeal against TfL.
The ride-hailing giant has been granted a new licence to work in the capital, nearly a year after TfL rejected its application, also over safety concerns because of unlicensed drivers.
Westminster Magistrates' Court heard that 24 Uber drivers shared their accounts with 20 others which led to 14,788 unauthorised rides.