Boris Johnsons has said one in three people who test positive for coronavirus will experience no symptoms.

The prime minister said transmission of Covid-19 from people who are asymptomatic is one of its "most striking" features.

He said: "We must remember one of the most striking features of this virus is that it spreads invisibly from people who don't even have symptoms which is, in fact, around one in three of everyone infected."

The comments were made during a Downing Street press conference during which Mr Johnson urged people to "think hard" about Christmas.

He said the message was: "A smaller Christmas is going to be a safer Christmas and a shorter Christmas is a safer Christmas."

Mr Johnson also advised people to minimise contacts before seeing loved ones at Christmas, and to avoid travelling from high prevalence areas to ones that are not so badly affected.