It will be "basically impossible" to negotiate all aspects of the future relationship between the European Union and Britain by the end of this year, the head of the European Commission said on Wednesday, adding that both sides must pick priorities.
Speaking at the London School of Economics before a meeting with British Prime Minister Boris Johnson, Ursula von der Leyen cast doubt on his timetable for an agreement defining the long-term post-Brexit relationship by the end of 2020.
"The transition time is very, very tight ... so it is basically impossible to negotiate all that I have been mentioning, so we will have to prioritise," she said.
Johnson is expected to tell the new EU commission head - a pivotal figure in the upcoming negotiations - that Britain will not extend the transition period, and will not seek a deal based on close alignment with EU rules.
Von der Leyen said the first topics that would need to be addressed were the areas where there were no international trade treaties to fall back on.
"It is not an all or nothing thing, but it is a question of priorities," Von der Leyen said, adding that there could be no compromise on the EU priority to uphold the integrity of the bloc's single market and its customs union.
She suggested that progress in the talks should be reviewed in the middle of the year, "or before summer" ideally, with a view to deciding then whether or not more time was needed.
Britain leaves the EU on 31 January but, while no longer an EU member, an agreed transition phase means it will remain bound by all the bloc's rules and pay into the EU's budget until the end of the year.
Unless London asks for an extension of the transition period beyond 2020, trade relations between the EU and Britain from the start of 2021 will either be governed by whatever agreement can be hammered out by the end of this year, or World Trade Organisation rules.
The British government does not want an extension of the transition period and is in the process of writing that promise - made during an election campaign last month - into law, making it illegal for Johnson to ask for more time.
"Without an extension of the transition period beyond 2020, you cannot expect to agree on every single aspect of our new partnership," von der Leyen said.
She noted that the further Britain diverges from the EU, the less access it will have to the bloc's single market.
"Without the free movement of people, you cannot have the free movement of capital, goods and services," von der Leyen said.
She reiterated the EU wanted a new partnership with Britain under which there would be "zero tariffs, zero quotas and zero dumping" and that would address everything from climate action to data protection, fisheries to energy, transport to space, financial services to security.
She also stressed the need for a comprehensive security partnership to fight cross-border threats "ranging from terrorism to cyber-security to counter-intelligence".