A Labour government will pass an emergency “NHS protection” law if it wins the general election, to ensure that powerful US pharmaceutical companies cannot infiltrate the health service and dramatically force up the price of drugs, Jeremy Corbyn announces on Sunday.

Writing in the Observer, Corbyn accuses Boris Johnson’s government of a covering up the details of secret talks that British and US officials have held to discuss greater access for American companies to the UK drugs market under a post-Brexit trade deal.

With Labour determined to put the future of the NHS centre stage in its election campaign, Corbyn demands the government come clean and immediately publish full details of the discussions. He describes them as part of a Tory plot with Donald Trump that will lead to the “runaway privatisation of our health service”.

It comes as Labour said it will give free dental checks to everyone in England, saving them a £22.50 charge. One in five people say they cannot afford to visit the dentist, and check-ups can detect tooth decay and oral cancer.

US government documents, made public earlier this year, spell out that the US’s ambition in any trade deal in the field of pharmaceuticals and medical devices is “full market access for US products”, raising the prospect of an end to current drug price controls in this country. Drug prices in America are on average 250% higher than in the UK.

The result, Labour believes, of allowing US companies into the UK market would be an extra cost of £500m a week to the NHS from increased charges – more than wiping out Johnson’s other NHS spending commitments.

“Mega-corporations see Boris Johnson’s toxic alliance with Donald Trump as a chance to make billions from sick people in Britain and around the world,” Corbyn writes.

Donald Trump and Boris Johnson: ‘Corporations see Johnson’s toxic alliance with Trump as a chance to make billions from sick people in Britain,’ Corbyn writes. Photograph: Evan Vucci/AP

“Johnson’s hard-right Brexit plans depend on a corporate-driven trade deal with Trump’s America.” Corbyn adds: “Our public services are not bargaining chips to be traded in secret deals. I pledge that a Labour government will exclude the NHS, medicines and public services from any trade deals – and make that binding in law.”

Last week the campaign group Global Justice Now published heavily redacted accounts of six meetings that have taken place between UK and US officials since 2017.

The documents were obtained following a Freedom of Information request that was initially refused. On appeal, the Department for International Trade released pages of blacked-out documents. A legal challenge for the full content to be revealed is due to be heard at the Information Rights Tribunal on 12 and 13 December.

Nick Dearden, director of Global Justice Now, said: “The UK government has gone to astonishing lengths to keep its plans for a Trump trade deal secret from the British public, and the absurd sight of page after page of blacked-out documents is a shocking display of their disregard for democratic accountability. Trade deals today are massive agreements that affect every aspect of our society, and it is unacceptable that Boris Johnson’s government refuses to tell the public what it’s putting on the table. Empty words are not enough – the government should be judged by its actions. These documents could reveal exactly what our government has already put on the table in trade talks. The public has every right to know what they contain before going to the polls on 12 December.”

Corbyn’s pledge to ensure legal protection for a fully publicly owned and run NHS in perpetuity came as senior Labour figures signed off the contents of the party’s election manifesto, which will be unveiled this week, probably on Thursday. The Conservatives have yet to say when they will reveal their manifesto, but last night they condemned Labour’s plan for free dental checks for all, saying they are “already free for those who don’t have the means to pay”.

Labour’s manifesto is expected to commit to another referendum on any Brexit deal it can negotiate, with the other option being to remain. Other likely commitments are to reduce tax loopholes available to private schools and an obligation on universities to take into account the backgrounds of applicants under a “contextual admissions” system, so they take into account the social backgrounds of applicants, including where they have been educated, rather than just exam results. The aim is to ensure that a greater proportion of applicants from state schools can get to university and reduce the advantages enjoyed by those educated in the private sector.

There will also be initiatives to extend workers’ rights, including an aspiration to reduce average working hours to 32 a week. Labour would also ban zero-hours contracts, which affect 900,000 people, and to give employees in companies with 250 or more staff the right to own shares under an “inclusive ownership fund” – benefiting 10 million people.

In the latest Opinium poll for the Observer the Conservatives are up three points compared with a week ago, and now stand on 44%. Labour is down one point on 28%. The Liberal Democrats are also down one point on 14%, their lowest showing since August, while the Brexit Party is unchanged on 6%. Brexit is cited by most voters as the most important issue, with 36% saying they will decide which party to support based on their approaches to leaving the EU. This is followed by health (20%) and the economy (7%).