The UK is facing its first significant test of how to contain a potential coronavirus outbreak after the government confirmed a 20th victim of Covid-19 and details began to emerge of its contingency planning and emergency legislation for an upsurge in cases.

The latest patient in England is the first to contract the illness while in the UK. The chief medical officer, Prof Chris Whitty, said it was unclear whether the man from Surrey contracted it directly or indirectly from someone who had recently returned from abroad.

“This is being investigated and contact tracing has begun. The patient has been transferred to a specialist NHS infection centre at Guy’s and St Thomas’ [hospital].”

The Guardian has been told there are now fears that a doctor from the patient’s surgery may also have been infected with the virus. If confirmed, this would prompt particular concern as the GP would routinely have seen scores of patients over the course of the last week.

As part of the official UK action plan being drawn up by ministers and Whitty, military medics, and British Red Cross and St John Ambulance personnel will be drafted in to help the NHS cope with a major outbreak.

Under ministerial planning for a “reasonable worst-case scenario” of a potential pandemic, doctors and nurses working for the armed forces would help at hospitals where staff who have the virus are too ill to work or are self-isolating at home.

The developments came as:

A British man became the first UK citizen to die from the virus after being infected on the Diamond Princess cruise ship in Japan. He was the sixth passenger from the ship to die, Japan’s health ministry said.

The government prepared emergency legislation to be introduced next week to ensure public services and the transport network can keep operating in case of a major outbreak. The laws include a measure to allow children to be taught in larger classes should teachers fall ill and another to speed up the disposal of bodies.

Stock market losses smashed through $5tn (£3.92tn) for the week, as the FTSE 100 and Dow Jones fell again.

The Bank of England governor, Mark Carney, warned that the expected global slowdown caused by the coronavirus fears will have a knock-on impact on the UK. Boris Johnson has said he would convene a Cobra meeting on Monday.

EasyJet and IAG, the owner of British Airways, both reported significant drops in demand and plans to cancelling flights.

The Geneva motor show was cancelled after Switzerland banned all gatherings of more than 1,000 people.

The International Olympic Committee member Dick Pound said a final decision on whether the Tokyo Olympics will go ahead in July could be made as late as May.

The World Health Organization said the risk of spread and impact of the virus is now “very high” after the Nigeria reported the case of the disease in sub-Saharan Africa.

Tracing the origins of the latest UK case will be crucial in preventing further spread, experts said on Friday night. Prof Jonathan Ball, a professor of molecular virology at the University of Nottingham, said: “This case – a person testing positive for novel coronavirus with no known link to an affected area or known case – marks a new chapter for the UK, and it will be crucial to understand where the infection came from to try and prevent more extensive spread.

“This was always a concern – this is a virus that frequently causes symptoms very similar to mild flu or a common cold, and it’s easily transmitted from person to person. This means it can easily go under the radar.”

In total there were four new UK cases on Friday, including the first confirmed positive test in Wales in a woman from Swansea. She had visited northern Italy, Europe’s worst-hit area, where there have been 21 deaths and 820 confirmed cases. Two new patients in England caught the virus in Iran and are being treated at a specialist unit at the Royal Free hospital in London, said Whitty.

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.

On Thursday Whitty said sports events and concerts may have to be cancelled, and schools closed for more than two months if the UK is hit by a global pandemic.

The exams regulator Ofqual is considering “contingency plans” over this summer’s exams in case of widespread outbreak.

In Surrey, health chiefs are worried about the possible diagnosis of the unnamed doctor of the new coronavirus case because his wife is also a GP. Public Health England is “contact tracing” everyone with whom either of the couple has been in contact – including all patients they have seen – so they can also be tested for evidence of the infection. It is not yet known if his wife has also contracted the virus.

Simon Lewis, the British Red Cross’s head of crisis response, confirmed that it had held talks with the government and NHS, and local council chiefs about what role it would play.

“We have already provided some assistance around isolation units at Arrowe Park hospital [in Wirral, where people have been quarantined after returning to the UK] and Heathrow airport, and we are talking to partners about any additional support we might be able to offer in the days and weeks ahead,” Lewis said.

There was also concern that news coverage of the virus might be prompting worried shoppers to start stockpiling food, with the online supermarket Ocado emailing a warning telling customers it was receiving high volumes of orders online.

It told customers “more people than usual seem to be placing particularly large orders” and urged them to place orders earlier than they usually would.