Donald Trump was warned yesterday to stay out of the General Election campaign ahead of his trip to Britain next week.
The Prime Minister was asked about Mr Trump's unprecedented intervention earlier this month, when he urged Mr Johnson to form a pact with Brexit Party leader Nigel Farage.
Mr Trump hailed Mr Johnson as "the exact right guy for the times" and warned that Labour's Jeremy Corbyn would take the UK to "bad places".
Speaking to LBC yesterday, Mr Johnson said: "When you have close friends and allies like the US and the UK, the best thing is for neither side to get involved in each other's election campaign." Britain is hosting Mr Trump and other Nato leaders for a two-day summit starting on Tuesday.
The US President is to visit Buckingham Palace for a reception with the Queen that evening, before he attends the leaders' meeting in Watford the following day.
Mr Johnson and Mr Trump are unlikely to have a bilateral meeting, but both will give separate press conferences on Wednesday. Asked if he would specifically meet Mr Trump, the PM replied: "I'm obviously going to be hosting the Nato leaders' meeting and look forward to meeting all the Nato heads of government coming to that meeting."
Senior Tories fear Mr Corbyn will try to exploit Mr Trump's visit to the UK next week to "peddle conspiracy theories" about the NHS.
Allies of Mr Johnson are braced for the President's arrival amid fears an intervention by him could derail the Prime Minister's election campaign.
They expect Labour to try to "weaponise" the NHS again following Mr Corbyn's failed attempt to claim it would be "sold off" in a trade deal with the US.
One Tory source said: "It will give Corbyn the chance to start peddling his NHS conspiracy theories again.
"We need to get the focus back to getting Brexit done."
Last month, in an interview with LBC, Mr Trump dismissed Labour's claims the US could undermine the NHS in a post-Brexit trade deal, insisting: "It's not for us." On Wednesday, Mr Corbyn made a great show of producing a 451-page document he claimed was "proof" of a plot to put the NHS up for sale.
The Tories accused Labour of "out-and-out lying", and analysis of the documents showed the NHS was mentioned only four times.
Mr Corbyn revealed the document in an attempt to distract attention from his disastrous BBC interview on Tuesday night in which he was exposed on tax, anti-Semitism and Brexit.
It comes as thousands of people, including NHS doctors and nurses, are expected to march to Buckingham Palace during the reception for Nato leaders on Tuesday.
The demonstration will be against Mr Trump's presence in Britain, Nato, and Mr Johnson's "inclusion of the NHS in trade talks with the US", the Campaign for Nuclear Disarmament said.
Wednesday's three-hour roundtable leaders' meeting in Watford will be kicked off with opening remarks from Mr Johnson and Nato Secretary General Jens Stoltenberg. Then there will be a closed session during which Mr Johnson and other leaders will make speeches.
There are likely to be tensions over Turkey's invasion of northern Syria, which strained relations between the Nato allies last month. They will discuss burden sharing, ensuring troops are ready to confront threats from Russia, space as a theatre of operation and counter-terrorism.
Last month French president Emmanuel Macron said Nato was "in the throes of brain death" and that European countries could no longer rely on the US to defend its allies.
Asked whether he still believed in the "collective defence" stipulations of Article 5 of Nato's founding treaty, under which an attack against one member is considered an attack against all members, Mr Macron said: "I don't know."