Uber went to court on Monday to overturn a decision stripping it of its license in London after being ruled unfit to run a taxi service in its most important European market.

Regulator Transport for London (TfL) shocked the Silicon Valley firm last September by refusing to renew its license, citing failings in its approach to reporting serious criminal offences and to background checks on drivers.

Uber, backed by Goldman Sachs and BlackRock among others and valued at over $70 billion, has faced protests, bans and restrictions around the world as it challenges traditional operators and angers some unions.

In London, the firm has made several changes to its business model since losing its license, including the introduction of 24/7 telephone support and the proactive reporting of serious incidents to the city's police. It has also changed its senior management and apologized for mistakes.

Uber's lawyer Thomas de la Mare said the company accepted the September ruling and the focus of the appeal should be on the reforms it had made since then.

"It's no part of our case to say it was wrong. We accept it was right," de la Mare told Westminster Magistrates Court in London. "It's that acceptance that has led to wholesale change in the way that we conduct our business."

While the appeal process is ongoing, Uber can continue to operate in London.

The appeal is due to be heard over three days and will hear from witnesses including Uber's UK Chairman Laurel Powers-Freeling, UK Head of Cities Fred Jones and TfL's Interim Director of Licensing Helen Chapman.

After its application for a five-year license was rejected last year, the company is now seeking an 18-month one to prove to the authorities that it has reformed.

Judge Emma Arbuthnot may take weeks before making her decision, which is likely to be subject to further appeal by the losing side, meaning the whole legal process could take years.

However, she signaled any renewal of the license could be for a shorter period.

"I would've thought, if I were to renew the license, 18 months would be rather too long," she said.

At stake for the U.S. firm is one of its most crucial foreign markets. Of its over 60,000 drivers in Britain, about 45,000 are in London.

Since September's TfL decision, Uber has also been stripped of its license by the southern coastal city of Brighton, in a decision which it is appealing, and the northern city of York.

It has, however, gained new licenses in Sheffield, Cambridge, Nottingham and Leicester.