Labour has obtained official documents showing that the US is demanding that the NHS will be “on the table” in talks on a post-Brexit trade deal, Jeremy Corbyn has said.

The Labour leader said the uncensored papers gave the lie to Boris Johnson’s claims that the NHS would not be part of any trade talks, and revealed that the US wanted “total market access” after the UK leaves the EU.

“The uncensored documents leave Boris Johnson’s denials in absolute tatters,” he said at a news conference in London. “We have now got evidence that under Boris Johnson the NHS is on the table and will be up for sale. He tried to cover it up in a secret agenda and today it has been exposed.”

Corbyn said the 451 pages of documents covered six rounds of talks from July 2017 to “just a few months ago”. He said the meetings took place in Washington and London. “We are talking here about secret talks for a deal with Donald Trump after Brexit,” he said.

Responding to the Labour claims, the Conservatives said the documents had already been online for two months and were simply readouts from meetings of the UK-US trade and investment working group. The Tories accused Labour of using the documents to try to divert attention from the issue of antisemitism in its ranks.

On medicine pricing, Corbyn said discussions had already been concluded between the two sides on lengthening patents. “Longer patents can only mean one thing: more expensive drugs. Lives will be put at risk as a result of this,” he said.

He used the example of Humira, used to treat Crohn’s disease and rheumatoid arthritis. “It costs our NHS £1,409 a packet. In the US, the same packet costs £8,115. Get the difference: £1,409 in our NHS, £8,115 in the USA,” Corbyn said. “One of the reasons for US drug prices being on average 250% of those here is a patent regime rigged for the big pharmaceutical companies.”

He added: “Let’s be frank, the US is not going to negotiate to sell its own medicines for less.”

Labour is battling to bring the focus back on to safer ground after Corbyn’s handling of antisemitism came under renewed criticism on Tuesday.

Corbyn avoided apologising to Jewish communities in a TV interview after the chief rabbi, Ephraim Mirvis, alleged he had let the poison of antisemitism take root in the party.

Corbyn had previously highlighted heavily redacted documents obtained by Labour campaigners relating to private meetings between UK and US officials discussing health being included in a trade deal. It is understood the party obtained unredacted versions in the last couple of days.

Corbyn said the documents revealed that the UK and US were closer to a deal than hoped. “[Officials] are ready to ‘exchange text’, which is trade-negotiator-speak for it being at a very advanced stage,” he said. “And they say they are ready to, I quote, ‘really take significant further steps’.”

Corbyn said the report from the third meeting said “everything is included [in trade talks] unless something is specifically excluded” and that the US wanted “total market access” as the “baseline assumption of the trade negotiations”.

He said officials had discussed a system to give corporations the power to sue the UK. “This is not only a plot against our NHS,” said Corbyn. “It is a plot against the whole country.”

He pointed to a passage in the documents that suggested the US would prefer a no-deal Brexit. “There would be all to play for in a no-deal situation but UK commitment to the customs union and single market would make a US-UK [free trade agreement] a non-starter,” it reads.

The Conservatives said it was simply fact that it would not be possible to strike a free-trade deal with the US if the UK remained in the single market and customs union.

The international trade secretary, Liz Truss, said: “Jeremy Corbyn is getting desperate and is out-and-out lying to the public about what these documents contain.”

She said it was Corbyn’s belief in “conspiracy theories” that had led him to fail to crack down on antisemitism in his party, pointing to reports that he had called on “western governments” to confront “the Zionist lobby” in a piece written for the Morning Star in 2011.

“People should not believe a word that he says, this stunt is simply a smokescreen for the fact that he has no plan for Brexit and that he has been forced to admit that he wants to increase taxes for millions of families,” she said.

“As we have consistently made clear, the NHS will not be on the table in any future trade deal and the price that the NHS pays for drugs will not be on the table. This sort of conspiracy theory fuelled nonsense is not befitting of the leader of a major political party.”

Labour’s manifesto includes a pledge to increase NHS funding by an average of 4.3% every year of the next parliament, which is more generous than Conservative and Liberal Democrat proposals.

The party has also promised to “end and reverse privatisation in the NHS in the next parliament”, as well as offering free annual NHS dental checkups and a new national care service to tackle the social care crisis.