The European Union (EU) is on course for a clash with British Prime Minister Boris Johnson after Commission President Ursula von der Leyen warned it will be “impossible” to get a full deal before his year-end deadline.

In her first major intervention on Brexit since she took up her post on December 1, Von der Leyen said the EU is “ready to work day and night” to strike a deal but warned Johnson that there isn’t time for detailed negotiations on all aspects of the UK’s future relationship with the EU.

The time is “very, very tight,” Von der Leyen told an audience at the London School of Economics ahead of a meeting with Johnson later in the day. “It’s not all or nothing, it’s a question of priorities,” she said.

Despite the mutual desire to present a clean slate after nearly three years of bad-tempered and tortuous negotiations on the UK’s withdrawal from the EU, both sides are digging in as Britain prepares to leave the bloc on Jan. 31. Buoyed by last month’s election victory, Johnson wants an ambitious free-trade agreement and has ruled out keeping the UK tied to EU laws beyond Dec. 31. His office said he’ll tell her that voters expect negotiations to conclude on time by the end of 2020.

But the next set of negotiations, on the type of relationship the U.K. and EU will have, could be even trickier than settling on the terms of Britain’s withdrawal. The future agreement should cover areas as diverse as the trade in goods and services, security cooperation, data sharing, fishing quotas and the terms under which European nationals can work in Britain.

“It is basically impossible to negotiate all of what I’ve mentioned,” she said.

Von der Leyen reiterated a message that the EU has given since even before the first Brexit talks started: the bloc has red lines on which it simply won’t budge.

“Without the free movement of people, you cannot have the free movement of capital, goods and services,” she said. “Without a level playing field on environment, labor, taxation and state aid, you cannot have the highest quality access to the world’s largest single market.”

As von der Leyen emphasizes the limits of what the UK can expect, some of Johnson’s supporters are looking for him to put more pressure on the EU by opening trade negotiations with other countries like the US. Johnson’s spokesperson James Slack told reporters on Monday that the UK would be able to start such talks after it has left the EU on January 31.

“We are ready to design a new partnership with zero tariffs, zero quotas, zero dumping,” Von der Leyen said. “A partnership that goes well beyond trade and is unprecedented in scope.”

“We should be optimistic,” she added. “We need to be optimistic.”