LONDON (Reuters) - Britain will find it “basically impossible” to negotiate all aspects of its future relationship with the European Union by the end of this year, the head of the European Commission said on Wednesday, adding that both sides must pick priorities.

European Commission President Ursula von der Leyen holds a news conference after a college meeting of the EU executive to discuss Iran crisis, in Brussels, Belgium January 8, 2020. REUTERS/Francois Lenoir

Speaking at the London School of Economics before meeting 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 prioritize,” she said.

Johnson has said Britain will not extend the transition period, and will not seek a deal based on close alignment with EU rules, although his spokesman said trade talks did not need to be completed all at once.

Von der Leyen’s views are widely shared in the EU. Croatian Prime Minister Andrej Plenkovic, whose country holds the rotating six-month presidency of the bloc, told reporters that Britain had to be sensible.

“We should adopt a negotiating framework which is inclusive but to approach the negotiations in a realistic manner,” he said in Zagreb. “That means not to put on a plate too many dossiers which we might not be able to deal with in the allotted time.”

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, 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.

Johnson’s office said an initial meeting between the British leader and von der Leyen had been positive, but said he restated that there could be no extension of the end-2020 deadline for a deal.

Britain would retain high labor and environmental standards after Brexit but would not accept alignment with EU rules or any jurisdiction by the European Court of Justice, a spokesperson for Johnson added.

TRANSITION PERIOD

Britain leaves the EU on Jan. 31 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 Organization rules.

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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 and counter-intelligence.