London Mayor Sadiq Khan said the lockdown announcement was “inevitable” and the only way to tackle the “out of control” level of Covid-19 cases.
“It is unclear why it took Boris Johnson so long to reach this conclusion,” Mr Khan said in a statement.
“I continue to call on the Government to make wearing masks mandatory outdoors when in queues, streets and other crowded spaces outside.
“To limit the risk of new variants spreading, we need more rigorous testing and quarantine rules put in place at our borders and airports.
“It is also crucial that greater financial support is provided to businesses, the self-employed and those self-isolating which is still woefully insufficient.
“Londoners have already made huge sacrifices, but once again I urge each and every Londoner to follow the rules. Stay at home, protect the NHS and help to save lives.”
However, Andrew Hayward, professor of infectious diseases epidemiology at University College London, said he is not sure if the announced lockdown will be enough to stifle the new coronavirus variant.
He told BBC Radio 4’s Today programme: “I think the lockdown announced yesterday will clearly save tens of thousands of lives.
“The threat we’re facing is at least as bad as we were back in March.
“I think the virus is different and it may be that the lockdown measures we had are not enough so we need to learn from the new insights and new technologies, we need to learn from the last lockdown and particularly some of the things we saw.
“I think this time round we really need to use this lockdown to bear down on the virus in a way that can protect key workers – for example, we could be using the lateral flow (tests) and working with employers to offer regular testing to key workers.
“We have millions of these and key workers will still be out there and we can protect them and reduce rates in key workers through that method, especially if we also make sure we pay for their isolation when they’re infected.”