LONDON      -         The president of the European Commission 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 concessions.

In a friendly but frank message to the U.K., Ursula von der Leyen said negotiating a new U.K.-EU trade deal will be tough. She also said the end-of-2020 deadline that British Prime Minister Boris Johnson has imposed on negotiations makes it “basically impossible” to strike a comprehensive new agreement in time.

Von der Leyen, who took over as head of the EU’s executive branch on Dec. 1, is visiting Johnson at 10 Downing Street in London later Wednesday for the first time since the British leader’s election victory last month.

Johnson’s Conservatives won a substantial parliamentary majority in Britain’s Dec. 12 election, giving him the power to end more than three years of wrangling over Brexit and take the U.K. out of the EU on Jan. 31. It will be the first nation to ever leave the bloc.

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 a new trade arrangement.

Johnson says the U.K. is seeking a free trade deal, but doesn’t want to agree to keep EU rules and standards. Britain wants to be free to diverge from EU regulations in order to strike new trade deals around the world.

Downing St. said when Johnson meets von der Leyen, he “will likely underline that the upcoming negotiations will be based on an ambitious FTA (free trade agreement), not on alignment.”

That could cause problems. Speaking at the London School of Economics before her meeting with Johnson, 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.”

“With every choice comes a consequence. With every decision comes a trade-off,” she warned.

International trade agreements typically take years to complete, but Johnson has ruled out extending the post-Brexit transition period beyond the end of 2020, although the EU has offered to prolong it until 2022. Downing Street said Wednesday that “both British and EU citizens rightly expect negotiations on an ambitious free trade agreement to conclude on time.”