U.S. President Donald Trump may find himself boxed out of a key trade deal being pushed by Republican lawmakers as Britain and the European Union begin what promises to be a lengthy and contentious series of negotiations following the country's formal exit from the bloc.

Lost in the pomp and ceremony of Britain's Article 50 triggering -- which came complete with photo-ops, Parliamentary statements and televised reactions -- is what could be a simmering dispute in early negotiating positions between London and Brussels. And one that could have big implications for Trump's trade ambitions. 

European Union officials threw down an early gauntlet Wednesday with a leaked document indicating they will not agree to a free trade deal until the two-year negotiation process has ended. The nine-page paper, seen by TheStreet, says "the United Kingdom will continue to enjoy its rights as a Member State of the European Union until the withdrawal agreement comes into force and will therefore also remain bound by its duties and commitments arising therefrom."

In other words, under existing rules, Britain can't cut a new deal while it's extracting itself from its current one. 

"What we should stress today is that, for now, nothing has changed," Tusk added in a formal reply to the Article 50 letter. "EU law will continue to apply to, and within, the United Kingdom."

The not-so-subtle broadside contrasts sharply with U.K. Prime Minister Theresa May's insistence that "we believe it is necessary to agree the terms of our future partnership alongside those of our withdrawal from the EU", a stance that was repeated no less than four times in her Article 50 letter.

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May's bold decision to meet with Trump earlier this year in Washington and invite the President -- who remains a significantly divisive figure in Britain -- for a state visit later this year expended a worrying amount of political capital for the Prime Minister, who has struggled at times to keep warring "hard" and "soft" Brexit factions within her party unified. 

Given that tension, May is keen to demonstrate Britain's attractiveness as a trade partner untethered to the European Union and a bi-lateral deal with the world's biggest economy would go a long way towards ensuring the country is "open for business" as it frequently insists.

Trump's desire to ink a new trade agreement -- on his own terms -- is no less compelling, particularly after his failure to push through health care reform and the various judicial challenges he's faced in addressing border controls.

He's also getting pressed by GOP lawmakers, including Charles Dent of Pennsylvania's 15th district, who, along with Mark Walker of North Carolina, are urging the "U.S. Trade Representative to begin negotiations with the United Kingdom for a new U.S.-United Kingdom North Atlantic Trade and Investment Partnership," according to a House Resolution introduced in January.

Should May cede to the rules already in place, she'll have to put a U.S. trade deal on ice until at least 2019 -- and possibly beyond -- and risk the ire of a President who's not especially known for forgiveness.

But if she violates her country's pact with the EU and starts trade talks with Washington before the, she risks alienating market that accepts around 45% of British exports (£220 billion, at last count) and makes 53% of the stuff the country imports.

We'll watch the talks with interest -- but also with a keen eye on a certain leader's Twitter feed.