“And given the chance, they’ll water down the rules on air pollution and our environment that keep us safe. They want a race to the bottom in standards and protections. What Boris Johnson’s Conservatives want is to hijack Brexit to unleash Thatcherism on steroids.”

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Corbyn was referring to America’s “defect levels” handbook that sets the maximum amount of contamination allowed in food – for example ground paprika must not have an “average of more than 11 rodent hairs per 25 grams”, and citrus juices “1 or more maggots per 250ml”.

EU rules do not have an accepted level of contamination, though in practice producers are just required to take all reasonable steps to avoid it.

Corbyn also warned that level playing field provisions in a trade deal with the US would cost the UK’s National Health Service an extra £500 million ($914 million) a week if medicine prices were set the same as in the US.

The figure comes from an independent report by a University of Liverpool academic, though it was described as a “worst-case scenario”.

A Channel Four investigation found that the UK’s caps on drug pricing had been discussed in initial trade talks between the UK and US, and US trade negotiators are likely to argue NHS policy on medicine purchases is discriminatory against US pharmaceutical companies.

Boris Johnson has insisted the NHS is “not on the table” in post-Brexit trade talks, and has also promised that food and safety standards would not fall.

In a surprise appearance on Nigel Farage’s radio show last week, US President Donald Trump said “I don’t even know where [it] started with respect to us taking over your healthcare system. I mean it’s so ridiculous. I think Corbyn put that out there, but to even think, it was never even mentioned, I never even heard it until I went over to visit with the Queen”.

Trump has previously said “everything is on the table” in a trade deal, then later said the NHS was not on the table.

Also on Monday the Liberal Democrats staged their campaign launch, with leader Jo Swinson insisting she was a genuine candidate and chance for prime minister. She has promised to cancel Brexit immediately if she won.

Swinson said “when I look at Boris Johnson and Jeremy Corbyn I am absolutely certain I could do a better job than either of them”.

Johnson was not fit to be prime minister because he “has always put his own interest ahead of the nation’s interest”, had lied to the Queen, to parliament and to the country, she said.

“With his big lies, brash promises and attitude to women it’s clear who he’s modelling himself on. Britain deserves better than Britain’s Donald Trump.”

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And she attacked Corbyn for “not giving you a straight answer” on Brexit, for backing another Scottish independence referendum and for failing to tackle anti-Semitism in his party.

Corbyn “should not be anywhere near Number 10”, she said.

Swinson has complained of being omitted from an upcoming TV leaders’ debate.

“It is ridiculous to suggest the country is well served by a debate between two men who want to leave the EU, squabbling about how to do it,” she said.