The virus is "spreading rapidly in every corner of our city," Sadiq Khan said on Thursday in front of the 25-strong London Assembly that is elected to hold the mayor accountable.

The new restrictions ban millions of people in the capital from meeting people from other households indoors.

The move applies for homes as well as venues like pubs and restaurants.

"Nobody wants to see more restrictions but this is deemed to be necessary in order to protect Londoners' lives by myself, London council leaders and by ministers," Khan said.

Londoners were also advised to avoid public transport and, if possible, reduce the number of journeys they make.

"I must warn Londoners that we've got a difficult winter ahead," Khan said.

Outdoor meetings with up to six people are still allowed.

Several London boroughs have in recent days exceeded the threshold of 100 coronavirus cases per 100,000 inhabitants.

With the new measures, the capital has moved to the second highest level in England's three-tier warning system alongside other regions such as York and Essex, Health Minister Matt Hancock told parliament on Thursday.

"We must take firm and balanced decisions to keep this virus under control," Hancock said.

"Things will get worse before they get better but I know that there are brighter skies and calmer seas ahead," the minister added.

Khan - like opposition Labour party leader Keir Starmer and several health advisers - has called for a renewed counry-wide lockdown to contain the pandemic more effectively.

The government of Prime Minister Boris Johnson, however, has so far stuck to his regional three-tier system.

Australian Associated Press