LONDON (AP) — The European Commission's president warned Britain on Wednesday that it won’t get the “highest quality access” to the European Union's market after Brexit unless it makes major compromises.

Ursula Von der Leyen said negotiating a new U.K.-EU trade deal will be tough, and that “with every choice comes a consequence.”

Britain is due to leave the EU on Jan. 31 and begin an 11-month transition period while the two sides negotiate a new economic relationship. Prime Minister Boris Johnson says the U.K. is seeking a free trade deal, but doesn't want to agree to keep EU rules and standards.

That could cause problems. Speaking in London before a meeting with Johnson on Wednesday, Von der Leyen warned that “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.”

The European Commission president also cautioned that “you cannot expect to agree on every single aspect of our new partnership” by the end of 2020. Johnson insists he will not extend the transition period beyond Dec. 31.

THIS IS A BREAKING NEWS UPDATE. AP’s earlier story follows below.

British Prime Minister Boris Johnson is meeting a top European Union leader on Wednesday, stressing that he won’t extend a year-end deadline for the two sides to negotiate a new trade relationship.

European Commission President Ursula von der Leyen is due to visit Johnson at 10 Downing Street for the first time since the British leader’s election victory last month.

Johnson’s Conservatives won a substantial parliamentary majority, giving him the power to end more than three years of wrangling and take the U.K. out of the EU on Jan. 31.

Britain’s departure will be followed by a transition period in which the U.K.-EU relationship will remain largely unchanged while the two sides negotiate. Johnson has ruled out extending the transition period beyond its current expiry date of Dec. 31.

EU officials say it will be hard to strike a new deal in just 11 months. But Johnson’s office said Wednesday that “both British and EU citizens rightly expect negotiations on an ambitious free trade agreement to conclude on time.”

Once talks begin in earnest, tensions are likely to arise around Britain’s desire to diverge from EU rules and standards in order to strike new trade deals around the world. Downing St. says Johnson “will likely underline that the upcoming negotiations will be based on an ambitious FTA (free trade agreement), not on alignment.”

EU officials, however, stress that the two sides can’t have close relations unless Britain accepts a “level playing field” for U.K. and EU businesses.