London hospitals will run out of beds within weeks if the spread of coronavirus is not dramatically reduced, the capital’s mayor has warned as he declared a “major incident” across the city.

Sadiq Khan said Covid-19 cases were “out of control” and implored Londoners to stay at home “unless it is absolutely necessary for you to leave” in order to save lives and protect the NHS.

He said that in some parts of the capital one in 20 people has coronavirus – compared to the England average of one in 50 – while there are 35% more people in hospital with Covid-19 than at the peak of the pandemic in April.

He called for the closure of places of worship and for face masks to be worn routinely outside the home, including in crowded places and supermarket queues, in a bid to curb case numbers.

“We’re at risk of NHS hospitals running out of beds in the next couple of weeks if the virus continues to spread and people continue to be hospitalised,” Mr Khan told Sky News.

“You’ll be aware that across the country on average one of 50 people have this virus. In London, on average, it’s one out of 30. In parts of London it’s one out of 20.”

He added that police officers would begin to drive ambulances “over the course of the next few days” to help handle the increase in 999 calls to London Ambulance Service, joining firefighters who had been helping already.

City Hall said the ambulance service is now taking up to 8,000 emergency calls a day, compared with 5,500 on a typical busy day.

The declaration of a major incident means that events are beyond the scope of business-as-usual operations and require special arrangements to be implemented by one or more emergency responder agency.

Major incidents were declared following the Grenfell Tower fire, the terror attacks at Westminster Bridge and London Bridge and the Croydon tram crash.

London’s regional director of Public Health England Professor Kevin Fenton said the situation now is the “biggest threat our city has faced in this pandemic to date”.

“The emergence of the new variant means we are setting record case rates at almost double the national average, with at least one in 30 people now thought to be carrying the virus.

“Our NHS services are under immense pressure and currently another 800 people are being admitted to our hospitals every day. We know this will sadly lead to large numbers of deaths, so strong and immediate action is needed.

“In order to ease the burden on our hospitals, we must first stop the spread. That means we have to stay at home. Cut your contacts, reduce your movements, do as little as possible.

“A lot has been asked of Londoners over the past 12 months but your decisions and actions right now have never been more important.”

Deputy Assistant Commissioner Matt Twist, who leads the Metropolitan Police response to the Covid pandemic, said the announcement of a major incident was a “stark reminder of the critical point we are at”.

He said rule-breakers cannot continue to “feign ignorance of the risk that this virus poses or listen to the false information and lies that some promote downplaying the dangers”.

“There can be no doubt that right now we find ourselves at a serious and dangerous crossroads for London; everyone must look at this news and understand that our health service is nearing breaking point.

“I know Londoners will be shocked that officers are still dealing with a small selfish minority who think the rules don’t apply to them by holding house parties, large warehouse raves or other gatherings. These are creating breeding grounds for the much more transmissible variant.

“These rule-breakers cannot continue to feign ignorance of the risk that this virus poses or listen to the false information and lies that some promote downplaying the dangers. Every time the virus spreads it increases the risk of someone needlessly losing their life.”

A spokesman for the Royal College of Nursing London branch said the situation in the capital was “dire” and nurses were “running on empty”, adding: “It’s vital for all Londoners to practise social distancing, wear face coverings, and stay at home. Don’t let the sacrifice of nursing staff be in vain.”

Dr Simon Walsh, deputy chair of the British Medical Association’s consultants committee – who works in emergency care in London, warned the impact of the national lockdown would not be seen for at least a week.

He said: “We have a depleted workforce who are exhausted but doing all they can to keep the health system running and it is therefore crucial that we urgently get the vaccine into as many healthcare workers as possible. It is also imperative that they have access to suitable protective equipment.”

Dr Samantha Batt-Rawden, an intensive care registrar and president of the Doctors Association, tweeted: “We tried. We really tried. NHS staff pleaded with people that Christmas is not worth it.

“Now 1 in 30 people in London have Covid and ICUs are overwhelmed. My heart is broken.”

St George’s Hospital in Tooting, south London, has had to vastly expand its intensive care capacity and move staff without specialist training to high-dependency roles in an effort to tackle the workload.

The PA news agency was granted rare behind-the-scenes access on Wednesday to the coronavirus front line at St George’s, which has seen its number of coronavirus patients at least matching the first peak.

Dr Mark Haden, an emergency department consultant, said: “The hospital bed occupancy is very, very high, it has lots of Covid patients as inpatients at the moment. It’s very stressful for staff and that is starting to show.”

Elsewhere, new analysis showed more than half of all major hospital trusts in England currently have more Covid-19 patients than at the peak of the first wave of the virus.

In two regions – eastern England and south-east England – more than three-quarters of trusts are above their first-wave peak.