Brussels warns UK of 'difficult' Brexit trade talks

But freshly-elected Conservative British prime minister Boris Johnson has ruled out any extension request - although he has a track record of going back on promises, having previously promised to leave the EU on 31 October 2019 "do or die".

The UK will in fact now leave the EU on 31 January.

"Without the freedom of movement of people, you cannot have the free movement of capital, goods and services [and] without a level playing field on the environment, labour, taxation and state aid, you cannot have highest-quality access to the world's largest single market," von der Leyen stated in London - in her first visit there since becoming commission chief.

As a result, the EU and the UK will have to prioritise some elements of the deal, she warned.

"Each side would do what was best for them," but the German head of the EU made clear that any solution must uphold "the integrity of the European Union, its single market and its customs union".

"We are ready to work day and night to get as much of this done within the timeframe we have," von der Leyen added.

"None of this means it will be easy, but we start this negotiation from a position of certainty, goodwill, shared interests and purpose," she said.

In practice, the commission chief pointed out that the UK will have nine to 10 months to get a deal in time for it to be ratified by 31 December.

New partnership beyond trade

Von der Leyen insisted that the EU is ready to negotiate "a truly ambitious and comprehensive new partnership" with the UK, which includes a trade deal with "zero tariffs, zero quotas and zero dumpings".

The commission president has also decided to reach a new partnership "that goes well beyond trade and has an unprecedented scope" - covering "climate action to data protection, fisheries to energy, transport to space [and] financial services to security".

However, the relationship between the EU and the UK will never be the same after Brexit, she said, despite the fact that the UK and the EU will still share the same challenges, values, believes, history and geography.

"The truth is that our partnership cannot and will not be the same as before. And it cannot and will not be as close as before, because with every choice comes a consequence," von der Leyen admitted.

But Brexit does not only mark the end of something, it also marks a new phase.

"When the sun rises again on 1 February the EU and the UK will still be the best of friends and partners," von der Leyen said.

"This is the story of old friends and new beginnings," she added.