Sadiq Khan said he has been meeting with colleagues from the NHS, councils and the fire service about the rise in infections of the COVID variant, with more than 10,000 new cases reported nationwide today.

While no new measures have been announced, he said: "In the last 24 hours, we have had the largest number of new cases since the pandemic began, more than 26,000.

"Hospital admissions are going up but also staff absences are going up by massive levels, so I have taken the decision, along with our partners, to declare a major incident."

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What's a 'major incident'?

A major incident is declared when an event or situation with a range of serious consequences requires special arrangements to be enforced by one or more emergency response teams.

Those could be the police, ambulance service or fire brigade.

They have previously been called for such incidents as the Grenfell Tower fire in June 2017 and the terror attack at Westminster Bridge in 2017.

Mr Khan said: "This is a statement of how serious things are, but also it means rather than different public authorities working separately we will be working together through both the London Resilience Forum and also through the strategic co-ordinating group."

What 'Plan C' restrictions could we see if Plan B fails to curb the spread of Omicron?

London's latest COVID figures

Latest figures showed there were 1,534 COVID patients in hospital across London on 17 December - a rise of 341, or 28.6%, compared with a week earlier when there were 1,193.

Across the UK, 7,611 people are in hospital with the virus, a rise of 163 patients (2%) on the previous week.

Those are all COVID cases - not just Omicron.

Mr Khan said Londoners will soon see more places offering COVID vaccines.

"It is really important Londoners understand how serious things are. The best thing Londoners can do is to get both vaccines and the booster, they provide extra layers of protection," he said.

Omicron now London's 'dominant variant'

Earlier, Mr Khan spoke to reporters at Chelsea's Stamford Bridge football ground, which is being used as a vaccination centre this weekend, where he said he was "incredibly concerned" about the increase of Omicron cases.

He said Omicron is now the "dominant variant" in the city.

He also called on the government to provide financial support for businesses impacted by the spread of COVID-19 to avoid people losing their jobs or companies going bust.

He said over the last few weeks the impact of COVID on businesses has been "devastating" with theatres, live music events, and restaurants hit by an increase in cancellations.

"It is crucial - the government has got to provide a financial package of measures, grants, 100% business rates relief and more news in relation to furlough," Mr Khan said.

"If the government doesn't, you are going to see many businesses going bust and tens of thousands of people losing their jobs."

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It comes as almost 3,000 sites, including racecourses, football stadiums and Christmas markets, have been turned into vaccination centres this weekend.

This includes football grounds Anfield, Stamford Bridge and Wembley and three racecourses, Sandown Park, Bath, and Chelmsford.

Other venues open this weekend include the Christmas Market at Chester Cathedral, the Liverpool Christmas Ice Festival, Bluewater shopping centre in Kent and London's Oxford Street where there will be a vaccine bus.