A High Court judge rules today on the legality of minicab-hailing app Uber.
Transport for London (TfL) is seeking clarification as to whether the smartphone app breaks the law by operating in the same way as meters used by more strictly regulated black cabs to calculate fares.
Section 11 (1) of the Private Hire Vehicles (London) Act 1998, bars all private hire cars from being "equipped" with taximeters.
Uber uses GPS technology.
The San Francisco-based company records a car's location and the journey time via smartphone and feeds the information to servers in California, and the fare is calculated and relayed back to driver and passenger.
London is one of the world's major cities where Uber has caused controversy and triggered legal battles.
TfL and Uber argued at a one-day hearing earlier this month that the app was not a meter and therefore not unlawful.
But the Licensed Taxi Drivers' Association (LTDA), which represents a substantial proportion of the 25,000 licensed taxi drivers in London, asked the judge to rule it was a meter and ban its use.
The Licensed Private Hire Car Association (LPHCA) is backing the LTDA and says the app is "an attempt to circumvent the statutory prohibition" on minicabs using meters.
Black cab drivers say the app poses a risk to public safety and customers being overcharged, with no opportunity to challenge fares before the money is automatically taken out of their bank accounts.
Government figures show the number of minicabs in the capital has risen by over a quarter (26%) in the past two years, to 62,800.
An Uber spokesman said: "We believe the Uber app on a partner-driver's phone is not a taximeter, and TfL - the regulator - shares this view.
"We are looking forward to getting binding clarity on this issue in the High Court."
But the spokesman stressed whatever the outcome it would not affect Uber's ability to operate in London, even if adjustments had to be made.
Martin Chamberlain QC, appearing for TfL, told Mr Justice Ouseley at London's High Court: "TfL brings these proceedings in its capacity as the regulating authority for both private hire and (black cab) taxis in London.
"The object is simply to attain the answer to a difficult legal question that only the court can answer authoritatively."
London mayor Boris Johnson has said disruption from tech companies such as Uber is inevitable, and the best cities can do is "create a level playing field" for established trades such as black cabs.
Speaking in Tokyo at a UK embassy event promoting start-up firms, he said: "Once the toothpaste is out of the tube, that's it. We have to strike a balance."
Previously, Mr Johnson said in his weekly Daily Telegraph column that the app was "allowing private hire vehicles to behave like black taxis: to be hailed, to ply for hire in the streets, to do exactly what the law says they are not supposed to do".
But it would be "nuts" to ban the technology behind the app and called for Uber, private hire firms and black cabs to work together and "find a balance" for the benefit of London.
Uber has criticised separate proposals to tighten private hire regulation in London following TfL's launching of a consultation on a series of measures that would affect minicab drivers.
They include the introduction of an interval of at least five minutes between a booking and the start of a journey, to allow drivers to plan an appropriate route.
Other proposals involve a requirement for drivers to pass an English language test and a map reading assessment, while firms might have to operate a fixed landline telephone and accept bookings up to seven days in advance.
Uber has collected thousands of names from customers in a petition and warned the changes "will mean an end to the Uber you know and love today".