London - British Prime Minister Boris Johnson's plan to negotiate post-Brexit trade arrangements with the European Union by the end of this year is unrealistic, making an extension or partial agreement necessary, said the European Commission president.

"Without an extension of the transition period beyond 2020, you cannot expect to agree on every single aspect of our new partnership," Ursula von der Leyen said before holding talks with Johnson in London.

"We will have to prioritize," von der Leyen said in a speech at the London School of Economics and Political Science (LSE).

Negotiating a new partnership

Von der Leyen was accompanied on by the EU's lead Brexit negotiator, Michel Barnier, in her first official meeting with Johnson since his election victory in December. The two sides "looked at the year ahead [and] negotiating a new partnership," Barnier tweeted after the talks.

He said future relations between Brussels and London "will not be as close as now," echoing comments in von der Leyen's speech.

"Time is short. A new clock is ticking," Barnier wrote, adding that Brussels will focus on implementing Britain's withdrawal agreement and preparing negotiations on future relations.

The EU with and without Britain

Von der Leyen said the start of Britain's formal withdrawal from the EU on January 31 will be "a tough and emotional day."

She added that London and Brussels will remain "the best of friends and partners" after Brexit.

Agreement on future relationships

Johnson has insisted that he will not extend the Brexit process again and will negotiate an agreement on future relations with the EU by December 31, just 11 months after "Brexit day."

In Zagreb, Croatia's premier backed von der Leyen's assessment of the timetable. "This [Brexit] is clearly an unprecedented situation," said Andrej Plenkovic, the head of the most recent addition to the club of 28 EU states.

Plenkovic was speaking ahead of opening ceremony for Croatia taking over the EU's rotating presidency, which von der Leyen was scheduled to attend.

The EU should "approach negotiations in a realistic manner," Plenkovic said. "That means not to put on a plate too many dossiers that we might not be able to deal with in the adequate time."

Sources said Johnson would stress to von der Leyen "the importance of agreeing a confident and positive future relationship" with the bloc by the end of this year.

He was also expected to say that negotiations on future relations should be "based on an ambitious FTA [free-trade agreement], not on alignment [with EU rules]," the sources said.

According to a draft resolution to be voted on by the European Parliament, British concessions on the post-Brexit rights of EU citizens will determine the closeness of the future economic partnership.

EU rules continue to apply to Britain

Free movement of workers is a key part of the EU single market and the parliament wants to maximize the rights of EU citizens after Brexit, David McAllister, chair of the parliament's foreign affairs committee, told reporters in Brussels.

Britain will have to follow EU rules more closely if it wants greater access to its markets, said McAllister, a member of Germany's Christian Democratic Union (CDU).

McAllister suggested that Johnson's tight timescale for negotiations could only produce an outline agreement on future relations, and the risk remains that Britain could leave the EU without future arrangements in place at the end of this year.

Transitional arrangements will apply until the two sides reach agreement on future relations, so little will change after January 31 as EU rules continue to apply to Britain.