A study earlier this month showed that a nine-year-old British schoolboy who caught the virus on a skiing holiday in France did not pass it on to anyone despite coming into contact with more than 170 people.

The French epidemiologists who carried out the research concluded that "children might not be an important source of transmissions of this novel virus".

But on Tuesday, British experts questioned the Swiss advice and warned that there remained a lack of scientific data to make such a radical policy pronouncement.

Professor Russell Viner, the president of the Royal College of Paediatrics and Child Health, said: "We don't think that it would be a good idea for children to hug their grandparents in the UK without more data.

"We think that children probably transmit Covid-19 less than adults, but we need to be absolutely sure and we would need to have a lot more data on that, particularly because elderly grandparents are in the vulnerable group."

There are reports that some members of the Government's Scientific Advisory Group on Emergencies (Sage) increasingly think there is evidence that children do not pass the virus to each other. That has led to a call for schools to reopen by June.

Current UK Government guidance says that anybody aged 70 or over, regardless of their health, should "be particularly stringent in following social distancing measures".

On Tuesday, Matt Hancock, the Health Secretary, announced that anybody over the age of 65 with Covid-19 symptoms now had the right to a test for the virus in a final dash to meet the target of 100,000 daily tests by the end of the month.