U.S. President Donald Trump and Britain's Prime Minister Boris Johnson | Pool photo by Stefan Rousseau via Getty Images Boris Johnson holds late-night chat with Donald Trump US and UK leaders discuss need for unity on defense ahead of NATO leaders’ meeting.

LONDON — Boris Johnson and Donald Trump snuck in a quick private meeting Tuesday evening with little fanfare ahead of a NATO leaders' meeting in London Wednesday.

The U.K. prime minister and U.S. president discussed the importance of the military alliance and the need for unity to address evolving threats during a head-to-head in Downing Street.

Johnson wants to avoid appearing too close to his U.S. counterpart, who is deeply unpopular in Britain, for fear it could scupper his chances in the country's general election next week. That could explain why the meeting was not announced in advance.

Earlier on Tuesday, Trump said he would not comment on the election, saying he did not want to "complicate" matters — before promptly endorsing Johnson as "competent."

He also helped his ally out by saying he did not want the U.K. public health service to be part of a trade deal with the U.S. even if it were handed over "on a silver platter." Opposition Labour Party leader Jeremy Corbyn has been claiming the opposite, saying Trump wants U.S. firms to win private contracts in the National Health Service and push up drug prices.

“The prime minister met President Trump this evening at Downing Street,” a No. 10 Downing Street spokesman said. "They looked forward to tomorrow’s NATO leaders’ meeting and reflected on the importance of the alliance to our shared security.”

The spokesman added: “The leaders welcomed the recent increases in defense spending by NATO member states and agreed on the need for the alliance to be unified in the face of new and evolving threats.”

At the meeting in Watford on Wednesday, Johnson is expected to heap gushing praise on the 70-year-old NATO structure.

"The fact that we live in peace today demonstrates the power of the simple proposition at the heart of this alliance: that for as long as we stand together, no one could hope to defeat us — and therefore no one will start a war," he will say, according to excerpts from his speech released ahead of time.

But amid discussion of the need to reform the pact, he will add: "As allies and friends, we must never shy away from discussing new realities, particularly NATO’s response to emerging threats like hybrid warfare and disruptive technologies including space and cyber."

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