Jeremy Corbyn has accused Boris Johnson of wanting to sell off the National Health Service to US healthcare firms, in a tense clash during the UK’s first head-to-head election debate.

The Labour leader read from documents stipulating “full market access” as a negotiating objective for US healthcare firms and highlighted talks that had taken place between the government and US trade officials.

“What we know of the government’s proposals, what we know of what Mr Johnson has done, is a series of secret meetings with the US in which they are proposing to open up our NHS markets – as they call them – to American companies. Freedom information act requests were made in order to find out what happened at these meetings,” he said, waving a document he had taken into the debate.

“Every single line of this document, redacted out. A document here, of US-UK negotiations. Summary of specific negotiating objectives: full market access for US products to our National Health Service. You’re going to sell out our NHS to the United States and big pharma.”

The documents were first uncovered by investigative journalists at Channel 4’s Dispatches programme, who back in October identified six formal discussions with US trade officials.

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Mr Johnson denied the claims and said he was surprised how often the issue was raised.

“This is an absolute invention. It is completely untrue! There are no circumstances whatever in which this government or any Conservative government will be on the table in any grade negotiation. Our NHS will never be for sale and I’m amazed how often this comes up,” he said.

A Labour Party spokesperson said: “The big moment of the first half was Boris Johnson’s silence when confronted with papers from US trade talks highlighting his secret plan to sell out our NHS to US corporations.

“Boris Johnson’s sell out deal will lead to years of uncertainty and open up our NHS to takeover by American corporations.”

A Conservative spokesperson said: “Corbyn claims he wants to get Brexit sorted, yet he has no plan for Brexit. If he is elected Prime Minister he will dither and delay for months and months and then force the country though the chaos of another two referendums.