Boris Johnson has likened leaked documents showing the US and UK held talks about the NHS after Brexit to faked photos of UFOs.

For the first time, the prime minister appeared to suggest the dossier – which revealed exploratory talks about possible higher medicine prices outside the EU – may have been falsified.

When it was put to him that the dossier proved talks had taken place, he replied: “There are photographs that purport to prove that there are UFOs.”

Labour’s allegation that the NHS was at risk in the post-Brexit US-UK trade deal that Mr Johnson craves was a “scare story” put out at “every election”, Mr Johnson insisted, on ITV’s This Morning programme.

The prime minister also tried to deflect growing criticism of his past comments about Muslim women, gay men, single mothers and the working-class as old news.

The general election campaign was “not the time to talk about articles written quite a long time ago”, he insisted.

The sofa interview – which Mr Johnson agreed to do as he continues to duck a grilling by Andrew Neil – focused on the NHS, on both his disputed plans to extra nurses and the controversy over a US trade deal.

The Trump administration has demanded “full market access” in the NHS and wants to end the ability of Nice, which regulates prices in the UK, to block drugs it does not consider value for money.

Washington also wants to change patent law, potentially paving the way for US drug firms to demand higher prices for their medicines and over a longer period of time.

Challenged that the dossier showed the NHS was “on the table”, Mr Johnson insisted that was “complete nonsense” and gave a “rock solid guarantee” that no part would be sold off.

He also urged the public not to feel unsafe, after last week’s terror attack, because Britain was “one of the safest countries in the world”.

“Simple things like stopping the early release of serious and violent criminals, that's what we've got to do,” the prime minister said.

“We've got to stop people coming out early and then committing the same sorts of crimes again.”.

Mr Johnson said it was “quite incredible” that the London Bridge attacker Usman Khan did not have to see the parole board before being released.

He also defended the proposal of taxing would-be NHS workers coming from the European Union after Brexit.

Doctor Sara, a This Morning viewer, said a “nurse tax” – paying for visa fees, immigration and health surcharges – could cost families up to £10,000 in their first five years in the UK.

The Conservative leader said: "I don't quite know what Sara means but it is only fair that people who come to this country make use of our services, from wherever they come from, should make a contribution.