Ursula Von der Leyen has said negotiators will need to “prioritise” in order to achieve an EU-UK trade deal by the end of 2020.
The European Commission president said the EU was willing to sign off on a deal that was “unprecedented in scope” and that would go “well beyond trade”.
However, speaking at the London School of economics, she insisted that it was not achievable to agree such a deal in just 11 months.
“Without an extension of the transition period beyond 2020, you cannot expect to agree on every single aspect of our new partnership,” she warned.
Boris Johnson's Withdrawal Agreement Bill has been re-worked to "legally prohibit" MPs from extending negotiations into 2021.
During a Q&A session following her speech, Ms Von der Leyen added: “The transition time - it is very, very tight.
"I briefly touched on the topics we have to negotiate, and this was not even the whole list ... it is basically impossible to negotiate all of what I’ve been mentioning.”
She added: “We’re determined to work as hard as we can, as much as we can, but there are of course limits.”
The German politician added that her preference would be to take stock of the situation in the summer when negotiators “might together want to take a reconsideration of the time frame.”
She also warned against Mr Johnson’s threat to abandon EU rules after Brexit, saying the move would create a “distant” relationship.
“Our partnership cannot and will not be the same as before.
“It 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”.
Ms Von der Leyen is meeting with the Prime Minister in Downing Street today.
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