The leader of the free world reacted “impatiently” last night to confirmation by Theresa May that Britain will have a two-year transition deal with Brussels before leaving the EU.
The source said: “The President told me ‘They’ve got it all to play for. We can give them [the UK] the best deal but I’ve got to be around to do it’.”
Mrs May’s deal will see Britain remain under EU control until 2021, two years after formally quitting the bloc.
However, this will overlap with US elections in 2020 which Mr Trump has no guarantee of winning.
Mr Trump was given a “full briefing” of the Prime Minister’s speech in Florence, Italy, on Friday in which she officially made public the details of her Brexit proposal.
“His reaction was impatience,” said the source.
“His concern is the UK finds a way of being able to negotiate trade deals before the elections in 2020.
“A good deal with Britain isn’t just about the special relationship that he respects so much, to him it’s also about jobs.
“If the Democrats win, it’s pretty clear they won’t be as eager as the President to bind the countries closely together, or at least, not so quickly.
“They are likely to see Brussels as the place where all the action is, and Britain will be back to the back of queue.”
Earlier this year Democrats warned that Mr Trump’s eagerness for a bilateral trade deal with Britain could damage US relations with Europe.
“US-UK ties are unique but they need not be exclusive,” said Bill Keating, the lead Democrat on the foreign affairs committee.
“There is a danger in having a prioritised bilateral agreement and leaving the rest of Europe by the wayside including allies like Germany.”
US business was more positive about Friday’s speech, with the US-UK Business Council, which represents three million business, praising the transition period for “avoiding a sudden and damaging shock to trade and investment flows”.
But it warned: “With limited time remaining before the UK leaves the EU, the two sides must negotiate a high-standard trade and regulatory cooperation agreement that will minimise economic and social costs associated with the reset in UK-EU relations.”