London, Britain's coronavirus-related death toll officially passed the grim mark of 20,000 after another 813 people who tested positive for COVID-19 have died in hospitals in Britain, according to the Department of Health and Social Care.

In the face of the "once-in-a-century" global health crisis, Britain, among other countries, have pledged greater international cooperation to jointly fight the coronavirus pandemic, Xinhua news agency reported on Saturday.

The total number of COVID-19 deaths in hospitals reached 20,319 in Britain as of Friday afternoon, while 148,377 cases of COVID-19 have been confirmed in the country as of Saturday morning, said the health department.

At Saturday's Downing Street media briefing, Home Secretary Priti Patel said the entire nation was grieving as Britain passed another tragic milestone.

Without drawing on when the current restrictions on movement would be lifted, Patel said it was imperative that people continue to follow the rules designed to protect their loved ones.

National Medical Director of NHS (National Health Service) England Stephen Powis said Saturday COVID-19 is a "once-in-a-century global health crisis" and will continue to be something "we work through in the months ahead".

Previously, the government's Chief Scientific Adviser Sir Patrick Vallance said that keeping Britain's death toll below 20,000 would be "a good outcome in terms of where we would hope to get to with this outbreak".

At Saturday's press briefing, Powis explained: "When Sir Patrick Valance and I made that comment a number of weeks ago, what we were emphasizing was that this is a new virus, a global pandemic, a once-in-a-century global health crisis, and this was going to be a huge challenge, not just for the UK but for every country."

Britain now is the fifth country with an official number of coronavirus-related deaths exceeding 20,000, after the United States, Italy, Spain and France.

One of Britain's leading public health experts told Xinhua Saturday that the real death toll could be much higher than the official figure which does not take account of people who died in care homes or at home.

Professor John Ashton, a former president of the British Faculty of Public Health, said: "Essentially there are four separate epidemics running in parallel, in hospitals, care homes, prisons and in the home. The official figures issued relate mainly to hospital deaths, but the others are gathering momentum."

Professor Ashton warned that there was also a risk of a second wave of outbreak in the fall, but the number of overall deaths would be lower if measures were taken to contain the spread of the virus.

He cited the approach China has taken, such as home working as part of the efforts to contain the outbreak.

"If we can get 20 or 30 per cent of people to work from home, or high numbers working one day a week at home, it would lower the total number of people who catch the virus and who die," he said.

On easing of the current lockdown, Ashton said when people start to return to work, they should be regularly tested for the virus, and at the same time ensure older people were shielded.

"It is also vital to test and shield people working on the frontline in hospitals, care homes and in the community," he added.

Britain has become one of 20 countries and global organisations, including the World Health Organization (WHO), to pledge to give world access to new vaccines and treatments in the fight against COVID-19.

The pledge follows calls from Prime Minister Boris Johnson and senior government ministers for greater global cooperation in battle against coronavirus.

Britain is to co-host virtually on May 4 a new Coronavirus Global Response Summit, aiming to raise money to develop vaccines, treatments and tests to help contain the coronavirus pandemic, announced the government Friday.

The move is aimed at boosting global supply of a COVID-19 vaccine, once one is approved for use, to help prevent a second wave of the pandemic, according to a government statement.

Foreign Secretary Dominic Raab, also first secretary of state, joined the UN secretary general, WHO director general and the leaders from the 20 countries, to pledge support for the new "COV-access agreement".

A British government spokesperson said: "The UK is one of the biggest supporters of the global effort to find a coronavirus vaccine, providing 250 million pounds (US $309 million) to international research on the disease to the Center for Epidemic Preparedness Innovations."

Scientists at the University of Oxford and Imperial College London are leading Britain's efforts to develop a working vaccine. Oxford researchers have begun vaccine trials on humans on Thursday.