A High Court judge has reserved judgment on whether the controversial minicab-hailing app Uber is unlawful.
The case was heard on the same day as London mayor Boris Johnson suggested the law governing private hire vehicles in the capital is being "systematically broken - or at least circumvented" by Uber.
Transport for London (TfL) asked the judge to rule specifically on whether the smartphone app is in reality operating as a meter like those found in taxis to calculate fares.
The app uses GPS technology and connects to external servers to carry out calculations of how much a customer owes based on the time a journey takes and the distance covered.
Section 11 (1) of the Private Hire Vehicles (London) Act 1998, bars all private hire cars from being "equipped" with taximeters.
Both TfL and Uber argued that the app is not a meter and therefore does not breach section 11.
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 that they are unlawful devices for calculating fares.
The Licensed Private Hire Car Association (LPHCA) is backing the LTDA and says the app "is an attempt to circumvent the statutory prohibition on the use of a taximeter".
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.
"However, the outcome of the case would not affect Uber's licence in London, or its ability to operate here."
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."
The QC added: "TfL has formed a view. If the court takes a different view TfL intends to ensure the law as declared by this court is properly enforced."
The judge reserved his judgment at the end of a hearing which was expected to last two days but finished in one.
The judge said he was also hoping it would not take him long to hand down his decision.
Mr Johnson gave his private judgment on Uber in his weekly Daily Telegraph column under the headline: "Uber is doing a terrific job - but it has to play by the rules."
He said 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.
Black cab drivers - several of whom were in court today - have demonstrated in the capital against TfL's handling of regulation for private hire car companies, in particular Uber, and suggested there is a risk to public safety and customers being overcharged with no opportunity to challenge charges before they are automatically taken out of their bank accounts.
Uber itself has hit out against proposals to tighten private hire regulation following TfL's launching of a consultation on a series of measures that would affect minicab drivers in the capital.
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 could have to operate a fixed landline telephone and accept bookings up to seven days in advance.
Uber has urged customers to sign a petition - it has nearly 120,000 names so far - and warned the plans "will mean an end to the Uber you know and love today".
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.