Hospitals in south London are struggling to cope with a thirteenfold “explosion” in the number of patients left seriously ill with Covid-19, which has led to operating theatres being turned into makeshift intensive care units, the Guardian has learned.

Doctors in hospitals across the capital are dealing with rising numbers of patients, many of whom are being ventilated to try to keep them alive. Medics are warning that numbers are doubling every three or four days and that the NHS in London will be treating “thousands” of people with the disease within a fortnight.

On Thursday evening, Northwick Park hospital in north-west London was forced to declare a “critical incident” after running out of critical care beds. It asked nearby hospitals to look after patients as it could not cope with the Covid-19 patients it was receiving.

Unpublished NHS figures obtained by the Guardian show that the number of people confirmed or suspected to have Covid-19 being treated in an intensive care unit (ICU) in hospitals in south London rose from seven on Friday 6 March to 93 on Tuesday 17 March – a thirteenfold increase in 12 days.

“There’s been an explosion in the numbers that began last weekend and has continued all week, with more and more cases every day,” said one London intensive care specialist.

Doctors are worried that 86 of the 93 cases last Tuesday were on a ventilator, suggesting that Covid-19 was leaving many needing life-or-death care. On that day, none of the 93 were receiving level one critical care, in which a patient receives occasional oxygen, and just seven were classed as level two patients, where they receive non-invasive ventilation through a mask.

“A very high number of patients receiving critical care are needing level three care, in which they are anaesthetised and have a tube put down their throat and the ventilator takes over the work of their lungs while they are unconscious,” said one doctor.

Coronavirus UK: how many confirmed cases are in your area? Read more

Four of the 86 patients are receiving extra corporeal membrane oxygenation (ECMO) treatment at St Thomas’ hospital opposite the Houses of Parliament. That is the highest level of medical intervention the NHS can provide to try to keep alive someone whose lungs have failed. It involves taking blood out of their body, adding oxygen to it and putting it back in again.

Although 49 of the 93 were classed as “suspected” Covid-19 patients, all were expected to test positive for the disease once results had come back, the doctor added.

The figures are the first to underline the steep surge in cases over the last week in London, which is facing more stringent social distancing measures after accounting for about a third of the country’s coronavirus deaths.

The figures come from the seven hospital trusts that make up the NHS’s south London adult critical care network, including Guy’s and St Thomas’, King’s College hospital and the trusts in Kingston, Lewisham and Greenwich, and Epsom and St Helier.

Quick guide What to do if you have coronavirus symptoms in the UK Show Hide Symptoms are defined by the NHS as either: a high temperature - you feel hot to touch on your chest or back

a new continuous cough - this means you've started coughing repeatedly NHS advice is that anyone with symptoms should stay at home for at least 7 days.

If you live with other people, they should stay at home for at least 14 days, to avoid spreading the infection outside the home. After 14 days, anyone you live with who does not have symptoms can return to their normal routine. But, if anyone in your home gets symptoms, they should stay at home for 7 days from the day their symptoms start. Even if it means they're at home for longer than 14 days. If you live with someone who is 70 or over, has a long-term condition, is pregnant or has a weakened immune system, try to find somewhere else for them to stay for 14 days. If you have to stay at home together, try to keep away from each other as much as possible. After 7 days, if you no longer have a high temperature you can return to your normal routine. If you still have a high temperature, stay at home until your temperature returns to normal. If you still have a cough after 7 days, but your temperature is normal, you do not need to continue staying at home. A cough can last for several weeks after the infection has gone.

Staying at home means you should: not go to work, school or public areas

not use public transport or taxis

not have visitors, such as friends and family, in your home

not go out to buy food or collect medicine – order them by phone or online, or ask someone else to drop them off at your home You can use your garden, if you have one. You can also leave the house to exercise – but stay at least 2 metres away from other people. If you have symptoms of coronavirus, use the NHS 111 coronavirus service to find out what to do. Source: NHS England on 23 March 2020

The NHS and government have not published any figures showing how many people with Covid-19 are receiving critical care, even though Public Health England (PHE) now collects them each day from all 134 acute hospital trusts in England.

PHE and NHS England refused a Guardian request to see the latest data for both London and England as a whole because it was “still experimental” and involved some “quality” issues.

The surge is putting significant strain on hospitals across the capital. Doctors involved at a number of hospitals have given the Guardian vivid accounts of the pressures they and colleagues are facing.

“I had to turn my operating theatre into a four-bed ICU and we are intubating two or three patients every eight hours or so,” said a doctor at one London hospital. An intensive care unit consultant at another hospital said: “The place is in panic stations trying to cope safely with the numbers of patients coming out of our ears.”

A third, at a different hospital, said: “All of our high dependency units [for patients less seriously ill than those needing intensive care] are being used for intensive care. We have patients spilling over into the recovery areas.

Q&A How can I protect myself and others from the coronavirus outbreak? Show Hide The World Health Organization is recommending that people take simple precautions to reduce exposure to and transmission of the coronavirus, for which there is no specific cure or vaccine. The UN agency advises people to: Frequently wash their hands with an alcohol-based hand rub or warm water and soap

Cover their mouth and nose with a flexed elbow or tissue when sneezing or coughing

Avoid close contact with anyone who has a fever or cough

Seek early medical help if they have a fever, cough and difficulty breathing, and share their travel history with healthcare providers

Advice about face masks varies. Wearing them while out and about may offer some protection against both spreading and catching the virus via coughs and sneezes, but it is not a cast-iron guarantee of protection Many countries are now enforcing or recommending curfews or lockdowns. Check with your local authorities for up-to-date information about the situation in your area. In the UK, NHS advice is that anyone with symptoms should stay at home for at least 7 days. If you live with other people, they should stay at home for at least 14 days, to avoid spreading the infection outside the home.

“In our critical care unit we have about 20 intubated patients, who are a mixture of suspected and positive cases. Normally we would never have more than 10 intubated patients.

“It is a controlled explosion of cases … I think the hardest thing is that we cannot let families visit ICU. We have declared the whole thing dirty and Covid, and we have an isolated area for non-Covid patients. We have clustered them there and wear different protective equipment so as to not cross-contaminate,” the doctor added.

One warned that it was only a matter of time before hospitals in London ran out of intensive care beds and had to start making difficult decisions about which patients could go on a ventilator.

Doctors also raised concerns about widespread shortages of the personal protective equipment needed to treat Covid-19 patients, given how infectious the disease is. “We are out of visors and masks are constantly running out in different locations,” said one.

Speaking of the Northwick Park critical incident, a senior manager at another London trust said it raised serious concerns about the ability of the capital’s hospitals to deal with the surge in patients. “Given we’re in the low foothills of this virus, this is fucking petrifying,” the official told the Health Service Journal.

• This article was amended on 21 March 2020 to correct references in the standfirst and text to a “fifteenfold” increase in patient numbers when “thirteenfold” was meant.