Up to 10,000 people may already have coronavirus in Britain and many families should expect to lose loved ones before their time, Boris Johnson warned on Thursday as he set out measures less stringent than those taken by other countries.

Medical, scientific and public health experts criticised the steps outlined to tackle what the prime minister described as the “worst public health crisis for a generation”, including ending school trips abroad, stopping older and vulnerable people taking cruises and the use of self-isolation for a temperature and cough lasting four hours.

Schools are to remain open and large gatherings such as sports events and concerts will go ahead, amid concerns that moving too soon with stricter measures could bring public fatigue, though the prime minister said both measures remained under consideration.

The moves put England and Wales at odds with neighbouring countries. Scotland moved to ban gatherings of more than 500 people from next week, while the Republic of Ireland and France became the latest countries to announce the closure of all schools.

Johnson’s moves came as the stock market in London suffered its blackest day since 1987, with panic selling – triggered by Donald Trump’s surprise ban on travel between the US and the Schengen zone – surpassing the worst seen in the 2008-09 financial crisis.

Quick guide What are coronavirus symptoms and should I go to a doctor? Show Hide What is Covid-19? Covid-19 is caused by a member of the coronavirus family that has never been encountered before. Like other coronaviruses, it has come from animals. The World Health Organization (WHO) has declared it a pandemic. What are the symptoms this coronavirus causes? According to the WHO, the most common symptoms of Covid-19 are fever, tiredness and a dry cough. Some patients may also have a runny nose, sore throat, nasal congestion and aches and pains or diarrhoea. Some people report losing their sense of taste and/or smell. About 80% of people who get Covid-19 experience a mild case – about as serious as a regular cold – and recover without needing any special treatment. About one in six people, the WHO says, become seriously ill. The elderly and people with underlying medical problems like high blood pressure, heart problems or diabetes, or chronic respiratory conditions, are at a greater risk of serious illness from Covid-19. In the UK, the National health Service (NHS) has identified the specific symptoms to look for as experiencing either: a high temperature - you feel hot to touch on your chest or back

a new continuous cough - this means you’ve started coughing repeatedly As this is viral pneumonia, antibiotics are of no use. The antiviral drugs we have against flu will not work, and there is currently no vaccine. Recovery depends on the strength of the immune system. Should I go to the doctor if I have a cough? Medical advice varies around the world - with many countries imposing travel bans and lockdowns to try and prevent the spread of the virus. In many place people are being told to stay at home rather than visit a doctor of hospital in person. Check with your local authorities. 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.

Fears that the US president had turned the possibility of a global recession into a certainty sent share prices into a tailspin and by the end of a day of turmoil, the FTSE 100 index was down 639 points – a drop of almost 11%.

In other developments, the Electoral Commission urged the government to postpone local and mayoral elections in May, while next week’s planned Brexit talks in London were called off. Universities across the country began to cancel in-person lectures in favour of online teaching.

Thursday’s press conference was told that Britain, where the death toll had risen to 10 and where there are 596 diagnosed cases, is believed to be four weeks behind Italy, which has seen 1,000 deaths and more than 15,000 infections. In the UK, the infection is expected to build to a peak in May or June.

Chris Whitty, the chief medical officer, said that worst-case scenario planning projected that 80% of the country would contract the virus, with a 1% mortality rate. This equates to more than 500,000 deaths.

Speaking at a sombre press conference in Downing Street, the prime minister said measures to tackle “the worst public health crisis for a generation” could “cause severe disruption” to everyday life for months.

“It is going to spread further and I must level with you, I must level with the British public: many more families are going to lose loved ones before their time,” Johnson said, as two more fatalities from Covid-19 brought the death toll in Britain so far to 10.

Dismissing the belief that the virus is comparable to seasonal flu, he added: “Alas that is not right. Due to the lack of immunity, this disease is more dangerous,” with the elderly at particular risk. Entire families could be told to self-isolate if one of them falls ill, and people should start looking after elderly relatives and neighbours, he added.

Today in Focus Can the NHS cope with coronavirus? Sorry your browser does not support audio - but you can download here and listen https://audio.guim.co.uk/2020/03/12-76989-20200312_TIF_NHS.mp3 00:00:00 00:27:05

Ministers and health chiefs are bracing themselves for an imminent sharp rise in infections and deaths. Sir Patrick Vallance, the government’s chief scientific adviser, said between 5,000 and 10,000 people in the UK are thought to have the virus now, as it spreads undiagnosed.

From now on, anyone with symptoms of the coronavirus must stay at home, schools should not undertake trips abroad and those aged over 70 with underlying health problems should not go on cruises, he said.

However, the former health secretary Jeremy Hunt criticised the government for not doing more, saying that countries that had closed schools and “acted earlier on social distancing” had seen success in halting the spread of the virus. He said people would be concerned that the government had not adopted policies such as the banning of external visits to care homes. “I think most people’s priority is their elderly loved ones who are most vulnerable,” he said.

Experts also criticised the measures as too limited to have a major effect and inadequate, given the scale of the looming threat to health.

“I can’t see that any of these measures are going to have a big impact … none of that is really going to affect transmission in the UK,” said Paul Hunter, professor of medicine at the university of East Anglia.

Prof John Ashton, former regional director of public health for northwest England, said: “This is a kind of ragbag with no particular logic to it … they are issuing some semi-directive things but they are not really doing what we need to do, which is to mobilise and encourage communities, neighbourhoods, families to form their own plans for the next period in which the local situation will influence what happens – whether it’s not going out to eat, or stopping sporting events.”

Coming on the day that Ireland decided to close all schools and colleges until 29 March, Johnson’s decision not to dramatically upgrade the government’s response highlighted again the stark difference between the UK’s approach and that of other European countries, which have instigated tougher action.

The UK is among a dwindling number of European countries – others include Belgium and the Netherlands – not to have introduced at least a partial closure of its schools. The Dutch prime minister, Mark Rutte, announced measures including a ban on gatherings of more than 100 people yesterday, but said closing schools would do more harm than good at this stage.

Flanked by Vallance and Whitty, Johnson defended their tactics.

“The measures that I have discussed today … staying at home if you think you have the symptoms, your whole household staying at home, looking after the elderly – making sure the elderly and vulnerable stay at home – these are the three most powerful defensive lines,” he said.

But Hunter said older people had been “left out on a limb” by ministers not setting our more detailed plans to protect their health.

Martin Hibberd, professor of emerging infectious diseases at the London School of Hygiene and Tropical Medicine, said the UK’s response “has clearly not been sufficient”. He and other experts called for much more extensive testing and tracing of the contacts of those diagnosed with Covid-19.

However, there was broad support for schools staying open, given children’s low risk from the virus and the economic and practical impact of parents left unable to work because they were looking after their offspring.

Signs that financial markets were starting to freeze up in response to growing evidence of economic distress caused by the pandemic created echoes on Thursday of the near-collapse of the banking system in 2008 and prompted an emergency cash injection from America’s central bank.

But the action by the New York Fed was not enough to make good the damage caused by Donald Trump, and botched attempts by the European Central Bank to reassure investors proved counter-productive.

The prospect of a sharp fall in business and tourist travel prompted concern over the financial health of airlines and saw an 80% drop in the share price of Finablr, owner of the airport money business Travelex.

But the pain suffered by sectors seen as especially sensitive to travel restrictions and weaker consumer demand was only part of a wider selloff that saw a fall in the share price of every FTSE 100 company.

Since the markets started to take fright at the economic implications of Covid-19 in late February, the FTSE 100 has fallen by more than 2,000 points to 5,237 and seen its value plummet by £543bn.

• This article was amended on 17 March 2020. An earlier version implied in the standfirst that it was chief medical officer Chris Whitty who suggested that half a million people could die. In fact Prof Whitty made it clear that he did not want to put “speculative numbers” into the public domain.