London's Nightingale hospital has been reopened and is admitting patients to help with the coronavirus spread in the capital.
Medical director Dr Vin Diwakar said the facility at London's ExCeL Centre also had a vaccination centre on site.
It was placed on standby in May after fewer than 20 patients were treated following a grand opening on 3 April.
Dr Diwakar said the Nightingale was being used to treat non-coronavirus patients.
In the Downing Street press conference, he explained it was taking non-Covid patients to help free up beds in London's hospitals.
He said: "This means that hospitals have more beds to care for Covid-19 patients and for our very sickest patients. We cannot do this indefinitely.
"There comes a point where if the infection gets further out of control, more and more patients from London will need to be transferred elsewhere."
At the start of November, he said, London had 1,000 Covid-19 patients.
This increased four-fold to 4,000 on Christmas Day and has doubled to just under 8,000 today, with more than 1,000 of those on critical care, he told the press conference.
???If the infection gets further out of control, more and more patients from London will need to be transferred elsewhere???— BBC News (UK) (@BBCNews) January 12, 2021
Dr Vin Diwakar from NHS England adds the Nightingale hospital in London is now being used to treat non-Covid patientshttps://t.co/5BfKuHeuVr pic.twitter.com/bQxh3zkqNl
But Dr Diwakar said there was "hope", with one hall of the ExCel Centre having opened as London's first mass vaccination centre.
"I can tell you Covid-19 is a horrible, horrible disease that leaves so many, including young people, breathless and gasping for life," he said.
On Friday, the Mayor of London declared a "major incident" as he described the coronavirus spread in the capital as "out of control".
More than 120 firefighters and 75 Met Police officers have been drafted in to help the London Ambulance Service cope with demand.