Social media giant Facebook has closed its three London offices from Friday afternoon through Monday for deep cleaning, after a visiting Singapore-based employee tested positive for coronavirus, Variety has confirmed.

“An employee based in our Singapore office who has been diagnosed with COVID-19 visited our London offices 24-26 February 2020. We are therefore closing our London offices until Monday for deep cleaning and employees are working from home until then,” said the company in a statement.

Variety understands the employee was last in one of Facebook’s London offices on Feb. 26, though it is still unclear which office was impacted.

Individuals who had direct contact with the employee have been asked to self-isolate and be vigilant in monitoring any symptoms. The firm is also believed to be reaching out to staff and contingent workers in other offices who visited London over the last few days.

The number of U.K.-based coronavirus cases jumped from 115 to 163 on Friday, the Department of Health confirmed. The increase by 48 cases in a single day is the highest so far in the U.K. The country’s first virus-related death was recorded on Thursday when a woman in her 70s with other health issues died in a Reading hospital.

Two British Airways staff at Heathrow, one of the busiest airports in the world, have tested positive for the virus and are now in self-isolation at home. They are reportedly part of the baggage handling team.

The virus is spreading evenly around the U.K. London has the most number of positive cases, with 29, followed by 24 in the South East of the country.

Meanwhile, the U.K. Government has pledged a further £46 million ($59.8 million) towards developing eight possible vaccines for the virus, taking the total amount of government spending on coronavirus to £91 million ($118.4 million). In addition, a new rapid virus test that could provide results in 20 minutes is being developed at the Mologic laboratory in Bedfordshire.

Prime Minister Boris Johnson, who visited the lab on Friday said: “Keeping the British people safe is my number one priority, and that’s why I’ve set out our four-part plan to contain, delay, mitigate and research coronavirus.”

“We are ensuring the country is prepared for the current outbreak, guided by the science at every stage. But we also need to invest now in researching the vaccines that could help prevent future outbreaks.”

I’m very proud that U.K. experts – backed by government funding – are on the front line of global efforts to do just that.”

Director-General of the World Health Organization Dr Tedros Adhanom Ghebreyesus said: “The United Kingdom’s support for the global response to the new coronavirus outbreak will protect the health of people in many parts of the world, from assisting countries with fragile health systems to fuelling the fight to find a vaccine.”

The U.K. is due to host the Global Vaccine Summit in June.