Jeremy Corbyn is today poised to confront Donald Trump face to face over his fears that the US President wants to buy up parts of the UK’s NHS in a “devastating” post-Brexit trade deal.

With the future of Britain’s healthcare system at the heart of Labour’s election campaign and the polls beginning to narrow, party sources did not rule out Mr Corbyn seizing his one and only chance to challenge the US President in person.

This evening both men will be at a Buckingham Palace reception ahead of tomorrow’s Nato meeting in Hertfordshire to mark the alliance’s 70th anniversary. Party sources confirmed the reception, hosted by the Queen, will be the first time their paths will have crossed.

HeraldScotland: Camley's Cartoon: Will the NHS be handed on a plate to Donald Trump?Camley's Cartoon: Will the NHS be handed on a plate to Donald Trump?

The SNP’s Ian Blackford and the Liberal Democrats’ Jo Swinson turned down the invitation to attend, citing prior campaign commitments. One party source quipped: “They’re both washing their hair.”

Last night, Ms Swinson urged Boris Johnson to give a guarantee “our farmers and world-leading food standards will not be sacrificed on the altar of a Trump trade deal” as she warned the Prime Minister’s “desperation" for a trade deal with America risked UK farmers being undercut by low-standard US imports.

The Stop Trump campaign said NHS nurses and doctors would lead an anti-Trump demonstration outside Buckingham Palace to coincide with the reception.

Tory HQ is on tenterhooks that, despite Washington's assurance the President has no intention of “wading into” the UK election campaign, there is always a possibility, deliberate or otherwise, of him doing so. Mr Trump is due to hold a post-Nato meeting press conference on Wednesday afternoon.

Last night, both Mr Corbyn and Nicola Sturgeon expressed their deep fears about what a post-Brexit transatlantic trade deal could mean for the future of the NHS north and south of the border.

In a letter to Mr Trump, the Labour leader demanded he urgently clarify that the NHS was “genuinely off the table” in UK-US trade talks and it would not be exposed to higher costs from US drug companies.

He called for the US’s negotiating objectives to be changed to, among other things, exclude any reference to pharmaceuticals, rule out removing caps on the amount the NHS spends on branded medicines, and drop the demand for “total market access” to UK public services.

In his letter, Mr Corbyn referred to the release of unredacted documents concerning talks between the UK and US Governments, which he claimed showed, despite Mr Johnson’s assurances to the contrary, that the NHS was up for sale in a post-Brexit trade deal.

This, the Labour leader, told Mr Trump, had increased concern the NHS “could be opened up to irreversible privatisation”.

He said the General Election meant the “British public need urgent clarity that our NHS is genuinely off the table in UK-US trade talks and will not be exposed to higher costs from US drugs companies”.

Mr Corbyn’s letter coincided with fears expressed by the First Minister, who warned Scotland’s NHS would be “on the line” if the PM agreed to Mr Trump’s demands in a “desperate attempt to agree a post-Brexit trade deal”.

Noting how the OECD had pointed out how overall pharmaceutical expenditure in 2016 was two and a half times higher per capita in the US than in the UK, Ms Sturgeon warned that even a 20 per cent hike in drug prices would add £360 million to NHS Scotland’s drugs bill; equivalent to the cost of a fifth of qualified nurses working north of the border.

"With the Tories desperate for a trade deal with the US, we simply cannot trust them not to put our NHS on the line by signing up to the bargain basement conditions demanded by Trump,” declared the SNP leader.

She added the fact the Conservatives had point-blank refused to back the SNP's plan for an NHS Protection Bill to provide a legal lock against the NHS being included in trade deals “tells its own story”.

The US President, who was due to arrive in the UK late last night, has a track record of controversy when he visits Britain.

Downing St highlighted Mr Johnson’s comments in an LBC interview last week when he noted: “We have very close friendships and relationships with the United States at every level of government but what we don’t do, traditionally, as loving allies and friends is get involved in each other’s election campaigns."

A senior US official responded by saying Mr Trump was “absolutely cognisant of not wading into other country's elections”.

Today, the UK, France, Germany, and Turkey will discuss counter-terrorism and Syria at a meeting in Downing St followed by the Buckingham Palace reception, which later in the evening will itself be followed by another one, involving just Nato leaders, in No 10.

Tomorrow, they will hold their 70th anniversary meeting at a hotel in Hertfordshire, followed by a press conference with Mr Johnson and then a separate one with Mr Trump.

Asked about a specific PM bilateral with the President, Downing St said there were no details yet but Mr Johnson, as the host, would, of course, be meeting all 29 attendees.