The US president, speaking in London at the start of the North Atlantic Treaty Organisation summit, claimed he would "stay out" of the General Election campaign because he does not want to "complicate it".
"At the same time the leader of the Opposition, Jeremy Corbyn, suggested that we should comply with Moscow's wishes and send them samples from Salisbury so that they could go through their allegedly impartial tests".
Asked if the NHS could be on the table in post-Brexit trade talks, Trump said: "No not at all".
The Prime Minister was finally pictured shaking the United States president's hand at the two-day meeting of alliance leaders in Watford, near London, on Wednesday, but has ducked other one-on-one photo opportunities.
Labour leader Jeremy Corbyn alleges that once Britain has left the European Union, Johnson will seek a trade deal with the United States that would drive up the cost of drugs and open up the state-funded service to USA firms.
Nearly half of Brits think Boris Johnson is lying about protecting NHS from Trump. The British public are "firmly against" an NHS sell-off.
But Downing Street said the two men had a low-key, off camera meeting in No 10 during Tuesday night's reception.
But McDonnell said on Wednesday that "all the evidence that we've seen has demonstrated objectively that it is", despite the denials of Trump and Johnson. This is desperation by the Conservative Party.
NATO Secretary General Jens Stoltenberg, left, and British Prime Minister Boris Johnson, center, greet U.S. President Donald Trump during official arrivals at a NATO leaders meeting at The Grove hotel and resort in Watford, Hertfordshire, England, Wednesday, Dec. 4, 2019.
Speaking during a campaign visit to Birmingham, Mr McDonnell said: "Donald Trump has a passing relationship with reality and truth sometimes". You know that I was a fan of Brexit.
Johnson insisted the North Atlantic Treaty Organisation meeting had been "extremely practical and extremely harmonious".
Asked if he would be meeting Mr Johnson, the president said: "I will be meeting with him, yes".
President Donald Trump said on Wednesday he discussed with British Prime Minister Boris Johnson the North Atlantic Treaty Organisation military alliance and trade at a meeting a day earlier.
Mr Johnson is set to issue a new appeal for unity, following a spat between Mr Macron and Mr Trump, which saw the president branding his French counterpart "very nasty" for calling North Atlantic Treaty Organisation "brain dead".
Elsewhere on the campaign trail, Labour will vow to end "rip-off Britain" as party research alleges that the Tories have cost families almost £6,000 a year by failing to curb rising bills.
The US President predicted that "Boris Johnson will do well" in the upcoming election on 12 December.
The Tories insisted Labour's vow to reduce living costs "defies belief".
Shadow chancellor John McDonnell is to highlight the scale of inequality in the United Kingdom during a speech in Birmingham on Wednesday as Labour criticises the Conservatives for creating a "cost-of-living crisis".