The head of NHS England has told anti-lockdown conspiracy theorists that their “nonsense” could kill by influencing people’s behaviour to ignore restrictions.

Sir Simon Stevens said baseless claims that coronavirus is a hoax and that hospitals are empty are an “insult” to health staff who are “working their guts out” on 12-hour shifts to help people infected.

He branded the claims “a lie”, as Boris Johnson urged conspiracy theorists to “grow up”.

It comes following demonstrators by Covid deniers outside hospitals in England.

At a Downing Street press conference, Stevens said: “Let’s just be completely straightforward about it – when people say that, it is a lie.

“If you sneak into a hospital in an empty corridor at nine o’clock at night and film that particular corridor and then stick it up on social media and say ‘this proves the hospitals are empty, the whole thing is a hoax’, you are not only responsible for potentially changing behaviour that will kill people, but it is an insult to the nurse coming home from the 12 hours in critical care having worked her guts out under the most demanding and trying of circumstances.

“There is nothing more demoralising than having that kind of nonsense spouted when it is most obviously untrue.”

Addressing news journalists who have been reporting from inside hospitals, Stevens went on: “You are reporting what is actually going on. That is what people need to concentrate on.”

The prime minister added: “The kind of people who stand outside hospitals and say Covid is a hoax and this kind of stuff – really, I do think they need to grow up.

“You heard eloquently now from the head of NHS England the pressure the NHS is under and we’ve all got to do our bit responsibly to protect it.”