Anthony Gardner, who is leaving his Brussels post after three years, said the US president-elect's transition team had been asking EU officials which countries will follow Britain in voting to leave the 28-nation bloc.
"To think that by supporting fragmentation of Europe we would be advancing our interests would be sheer folly. It's lunacy," Gardner told reporters in unusually tough on-the-record remarks at the US embassy in the Belgian capital.
Leading Brexit campaigner Nigel Farage was the first British politician to meet Trump following the New York real estate tycoon's shock election victory in November, and has been back for a second visit.
French far-right presidential candidate Marine Le Pen, who has threatened a so-called "Frexit", was seen at Trump Tower in New York yesterday although officials said she did not meet the president-elect or any of his staff.
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"That was the one question that was asked, basically what is the next country to leave, which is kind of suggesting the place is about to fall apart," Gardner said.
"It's a perception that Nigel Farage is presumably disseminating in Washington. And it's a caricature," he added.
Gardner, a former private equity head in London appointed by President Barack Obama in 2013, said US policy for 50 years has backed EU integration because it benefits both sides for political, economic and security reasons.
"I believe some hold that view," he added, but despite a range of crises besetting the bloc, "the EU... Is not about to fall apart."
Gardner also hoped Trump, who has been seen as cozying up to Russian President Vladimir Putin, would stay tough on the US-EU sanctions imposed on Moscow since its 2014 annexation of Crimea from Ukraine.
"It would be inconceivable and shameful if we were to consider lightening sanctions on Russia," Gardner said, citing US intelligence claims of Russia's meddling in the US elections.
"That is a breach of precedent established over decades," he said.
He said he plans to speak out publicly about his concerns about populist threats to human rights and the rule of law as he stays on in Europe as a private citizen.