Ursula von der Leyen has warned that not every aspect of the EU and UK's future relationship will be able to be agreed on time if the Brexit transition period isn't extended beyond 2020.
The European Commission President was speaking in central London this morning, ahead of a meeting with British Prime Minister Boris Johnson later today.
Mr Johnson has brought forward legislation to rule out any extension on trade deal negotiations beyond the end of this year.
Those negotiations are set to get underway once the UK leaves the EU at the end of this month.
Today, Ms von der Leyen said the divorce negotiations were "done and dusted", and that she expected both the British parliament and the European Parliament to pass the withdrawal deal by the deadline of 31st January.
She said the UK's final day in the EU will be a "tough and emotional" one, but stressed: "When the sun rises again on February 1st, the European Union and United Kingdom will still be the best of friends and partners.
"We will still have a lot to learn from each other... We will still face the same challenges, from climate change to security."
'Cannot and will not be the same'
Ms von der Leyen pledged to create a "truly ambitious and comprehensive new partnership" with the UK.
But we are ready to design a new partnership with zero tariffs, zero quotas, zero dumping. A partnership that goes well beyond trade and is unprecedented in scope.
And we are ready to work day and night to get as much of this done within the timeframe we have. #LSEEurope pic.twitter.com/vMIRkROid8— Ursula von der Leyen (@vonderleyen) January 8, 2020
However, she warned: "The truth is that our partnership cannot and will not be the same as before.
"It cannot and will not be as close as before, because with every choice comes a consequence... with every decision comes a trade-off.
"The more divergence there is, the more distant the partnership will be - and without an extension of the transition period beyond 2020, you cannot expect to agree on every single aspect of our new partnership: we will have to prioritise."
Ms von der Leyen stressed that the EU was entering the negotiations with the EU "from a position of certainty, goodwill, shared interest and purpose" - adding that there was a "need to be optimistic."
She noted that the European Union must continue to "forge its own path in today's world" after Brexit.
She observed: "One consequence of the Brexit vote has been to strengthen the unity and the faith in Europe as a project for the common good.
"The truth is that Brexit has highlighted the value of being together in today's ever more unsettled world."
