President Trump on Thursday said his administration wants to make a "large scale trade deal" with the U.K. and called the potential for such a deal "unlimited" -- a boost to British Prime Minister Theresa May’s besieged government on the same day as lawmakers across the pond are due to vote on delaying Britain’s departure from the bloc.

“My Administration looks forward to negotiating a large scale Trade Deal with the United Kingdom,” he tweeted. “The potential is unlimited!”

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Trump’s pledge is likely to be welcomed by pro-Brexit lawmakers in the U.K., who have signaled the value of Britain being able to negotiate its own trade deals once it leaves the E.U. Under E.U. terms, individual countries are not allowed to negotiate their own deals.

Trump has been a vocal supporter of Britain’s 2016 decision to leave the E.U., claiming that the outcome of that vote foreshadowed his election victory a few months later. He has allied himself with pro-Brexit figures in the U.K., and has repeatedly promised to make a trade deal with the U.S. ally. Last summer the U.S. Trade Representative’s Office formed a trading “committee” with the U.K. to work out a trade agreement, as a way to get around the fact that the U.S. cannot formally negotiate with the U.K. until it leaves the E.U.

Later on Thursday, in the Oval Office alongside Irish Prime Minister Leo Varadkar, Trump said he was "surprised at how badly it's all gone from the standpoint of a negotiation" and said that May didn't listen to his advice on how to negotiate with the E.U. He also said that a second referendum, that some pro-E.U. politicians are calling for in the U.K., would be "unfair."

"I hate to see everything being ripped apart right now, I don't think another vote would be possible because it would be very unfair to the people that won that say: 'What do you mean you're going to take another vote?'" he said. "So that would be tough."

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Britons voted in 2016 to leave the E.U., and Britain is set to leave the bloc on March 29. But Parliament overwhelmingly voted down May’s withdrawal agreement this week for the second time, leaving Britain set to exit without a deal -- something that business leaders and pro-E.U. lawmakers have warned would be catastrophic for the country.

Pro-Brexit lawmakers have downplayed fears of chaos at ports and food and medicine shortages, noting that the U.K. will revert to World Trade Organization (W.T.O.) terms, and have dismissed the warnings as part of a “Project Fear” by those seeking to delay and eventually cancel Brexit.

Lawmakers voted Wednesday on a non-binding motion to reject a “no deal” Brexit, meaning that Parliament will vote Thursday on a motion that would call for May’s government to seek an extension of Britain’s departure until June.

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It is not clear that E.U. leaders will agree to such an extension. French President Emmanuel Macron said last month that E.U. leaders would only agree to an extension “if it is justified by new choices by the British.” European Council President Donald Tusk said on Thursday that he would call for E.U. leaders to be open to a “long extension” but that was on the condition that Britain “rethink its Brexit strategy and build consensus around it.”

Fox News' Edward Lawrence contributed to this report.