Hospitals in the British capital are being overwhelmed by COVID-19 patients, the head of an organisation representing bosses in the state-run National Health Service said on Thursday.

The chief executive of NHS Providers, Chris Hopson, told BBC radio that London hospitals had seen an "explosion of demand... in seriously ill patients", likening it to a "continuous tsunami", with numbers predicted to surge in the next fortnight.

"They talk about wave, after wave after wave," he said. "The word that's often used to me is a sort of continuous tsunami."

According to the latest figures, 463 people have died from the virus and more than 9,500 people have been infected. London makes up a third of the UK total. The government is opening a temporary 4,000-bed hospital at an exhibition centre in London next week to treat patients with the virus.

British media reported 10 similar facilities could be set up around the country.

Hopson said the problem of dealing with the increased number of patients was exacerbated by the "unprecedented absence rate" of medical staff. "We're now seeing 30, 40 and even some places 50 percent sickness rates," he said, adding that there was also a "real problem" with ventilator capacity in hospitals.

His comments came as the government announced it had ordered 10,000 emergency ventilators to help tackle the pandemic from billionaire entrepreneur James Dyson.

In an email to staff, the inventor said his company designed the "CoVent" at the request of Prime Minister Boris Johnson, and promised to donate 5,000 to the international relief effort. Dyson said teams of engineers had been working on the design since receiving the call 10 days ago.

"We have received an initial order of 10,000 units from the UK Government, which we will supply on an open-book basis," he said. "We are also looking at ways of making it available internationally."