LONDON -- The car hiring app firm Uber will not be issued a new private hire licence, Transport for London (TfL) said Friday.

TfL has concluded Uber was not fit and proper to hold a private hire operator licence after expiry of its current licence on Sept 30.

TfL considered that Uber's approach and conduct demonstrate a lack of corporate responsibility in relation to a number of issues which have potential public safety and security implications.

The transport regulator has questioned about Uber's approach to reporting serious criminal offences, how medical certificates are obtained, and how Enhanced Disclosure and Barring Service (DBS) checks are obtained.

It also said the taxi-hailing app uses Greyball, a software that could be used to block regulatory bodies from gaining full access to the app and prevent officials from undertaking regulatory or law enforcement duties.

Currently some 3.5 million passengers and 40,000 drivers use the Uber app in London. Opponents of the firm have claimed it causes gridlocked roads and does not do enough to regulate its drivers.

Mayor of London Sadiq Khan voiced his support to TfL's decision in a statement, saying that 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."

Uber said it will challenge the decision to keep the licence. It added that the decision shows the world that "far from being open, London is closed to innovative companies."

Uber has 21 days to appeal, during which it can continue to operate.

Uber London Limited was licenced as a Private Hire Operator in 2012. On May 26, TfL granted a 4-month licence while it concluded its consideration of a five-year licence.