London could face new restrictions banning different households from mixing indoors as early as this week, mayor Sadiq Khan has warned.

The capital is expected to be initially placed on medium in the new three-tier system of local alerts for England when they come into force on Wednesday.

This would put London with most of the country at the lowest end of the scale, meaning no new restrictions would be imposed immediately and the rule of six and the 10pm curfew staying in place.

But the London mayor warned that the city could be moved upwards “very quickly, potentially even this week” after the prime minister Boris Johnson set out his plans earlier today.

If the city moves up a notch to “high”, then mixing between different households or social bubbles could be banned indoors, including in pubs and restaurants.

A spokesman for the mayor said: “The virus is now spreading very quickly in every corner of London. The number of cases is rapidly increasing and all the indicators we look at are moving in the wrong direction.

“As of today, London is at ‘medium’ in the government’s new alert levels. However, Londoners should understand that this could change very quickly - potentially even this week.”

Khan discussed the new alert system with local leaders in the capital on Monday, and they are understood to have agreed further restrictions will soon be needed if the figures continue on the current trajectory.

His spokesman added: “Discussions with London leaders, scientific advisers and the government will continue over the coming days and the mayor will be as clear as possible with Londoners about what they can expect as soon as possible.

“Nobody wants to see new measures that will impact on businesses in London such as the hospitality sector - least of all the mayor - but we have a responsibility to act on the evidence if it will save lives.”