Jeremy Corbyn has said there would be “no deal” with the US, under a Labour government, if it insisted the NHS was included in post-Brexit trade talks.
The Labour leader has written to Donald Trump setting out his demands, ahead of the US president’s arrival in the UK for a Nato summit.
He said: “The British public need urgent clarity that our NHS is genuinely off the table.”
Boris Johnson has insisted the NHS is safe in the Conservatives’ hands.
The Conservative manifesto explicitly states that neither the price paid for drugs nor NHS services will be at stake in post-Brexit trade discussions with the US.
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Mr Trump’s arrival in the UK, for an event to mark the 70th anniversary of the transatlantic military alliance, comes at a crucial time in the general election campaign, with just over a week left until polling day on 12 December.
Liberal Democrat leader Jo Swinson is calling on Mr Johnson to use discussions with Mr Trump on the sidelines of the Nato summit to seek protections for British farmers and consumers.
Ms Swinson claims that the leaked documents from UK-US trade talks show American officials are pushing for Britain to allow greater use of chemicals in food production, such as chlorine-washing chicken and growth hormones in beef cattle.
Ms Swinson said: “Johnson’s desperation for a post-Brexit trade deal with Donald Trump means UK farmers risk being undercut by low-standard imports from the US.
“Boris Johnson must give a guarantee that our farmers and world-leading food standards will not be sacrificed on the altar of a Trump trade deal.”
Mr Corbyn has repeatedly claimed that the NHS would be “up for sale” if the Conservatives win the election – something the Tory leader has dismissed as “nonsense”.
Last week, Mr Corbyn called a press conference at which he brandished an unredacted report that gave details of meetings between US and UK officials, where the general requirements of a trade deal between the two nations after the UK leaves the EU were discussed.
It included confirmation of a round of meetings on patented medicines, where officials explained how drugs were approved for use on the NHS and described a US request for “total market access” to UK public services as a “baseline” for an agreement.
Mr Corbyn said this was evidence that US drugs firms were pushing for patents on their most popular drugs to be extended, thereby making it impossible for competitors elsewhere to produce cheaper, generic versions.
Speaking at an election rally on Monday evening, Mr Corbyn said the disclosures were “frightening”.
“My simple message to Boris Johnson and Donald Trump is that the NHS is not for sale,” he said.
The Labour leader has written to the US president saying any increase in the cost of NHS drugs as a result of US trade deal would be “unacceptable” to the British public.
He called on the US president, who he could meet at a reception at Buckingham Palace on Tuesday, to revise his negotiating objectives to accept the role of UK watchdogs in deciding the cost-effectiveness of medicines and to rule out including any patient data in any deal.
He is also pressing the US to rule out any dispute settlement mechanism by which the UK government could be sued for protecting public services, something he said US firms were already doing elsewhere.
Such guarantees, he said “would go a long way to reassuring the British public that the US government will not be seeking total market access to the UK public services… and that the US government accepts that our NHS is not for sale in any form”.
Mr Corbyn sent a letter with similar demands to Mr Johnson on Monday.
Speaking to the BBC, he said Mr Trump was welcome in the UK and he would be happy to engage with him.
“I talk to anybody. The whole point of political life is the ability to engage with others particularly where you may not initially see eye to eye – but persuasion is possible.”
But he said he would make clear the NHS and other public services, such as the care sector, could not form part of any future discussions “in any circumstances”. Asked what would happen if the US insisted on their inclusion, he replied “then there will be no deal”.
Mr Trump declined a meeting with the Labour leader during his state visit to the UK in July. As recently as last month, he said that Mr Corbyn as PM would be “bad for your country”.