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"We urge both the United Kingdom and the European Union in their negotiations to continue to be flexible and work this out in a process that is smooth, pragmatic, transparent and productive," spokesman Eric Schultz told reporters aboard Air Force One.

He noted that Washington was closely watching developments and working with officials in London and Brussels to ensure that Brexit doesn't upset financial markets.

Schultz also added that the "special relationship" between the United States and Britain endures.

"It is a bedrock of the United States' foreign policy. But likewise, the remains an indispensable partner for the United States in stimulating economic growth and addressing regional and global challenges."

Three senior judges on the British High Court ruled Thursday that Prime Minister Theresa May's government does not have the power on its own to trigger Article 50 of the European Union's Lisbon Treaty, the formal notification of Britain's intention to leave the bloc.