Britain will not get "the highest quality access" to European markets after it leaves the EU, unless it makes significant compromises, the European Commission's president warned on Wednesday.
The United Kingdom's future trade deal with the European Union will require tough negotiation, said Ursula von der Leyen on a visit to London, adding: "With every choice comes a consequence."
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UK Prime Minister Boris Johnson wants to agree on a new economic relationship with the world's largest trading bloc by the end of 2020 - but has signalled that he does not wish to follow EU rules and standards.
This could become an obstacle.
For Brexit supporters, one of the main reasons to leave the EU is to avoid regulation and red tape, which they say stifle business and enterprise.
Hardliners in Johnson's own party and other eurosceptics will not accept leaving the EU on paper while still being bound by its bureaucracy.
"Without a level playing field on environment, labour, taxation and state aid, you cannot have the highest quality access to the world's largest single market," warned Von der Leyen.
Trade negotiations can only formally begin once Britain leaves the EU on January 31, but 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 December 31 and has removed text from his proposed Brexit legislation that would allow MPs to delay the deadline if no deal is reached.
Johnson's Conservative Party won a substantial parliamentary majority in December, giving him the power to end more than three years of wrangling over the UK's withdrawal from the EU.
His office on Wednesday said: "Both British and EU citizens rightly expect negotiations on an ambitious free trade agreement to conclude on time."
Von der Leyen also said that ties between the UK and the EU will continue to be "unbreakable".
She told an audience at the London School of Economics, where she formerly studied, that many of the principles of the EU and the single market had been guided by British figures.
Von der Leyen said January 31 - Britain's exit day - would be a "tough and emotional day".
But she added: "When the sun rises again on February 1, the United Kingdom and the European Union will still be the best of friends and partners.
"The bonds between us will still be unbreakable."
