Prime Minister Boris Johnson has said everyone in the UK should now avoid “non-essential” travel and contact with others and to fight coronavirus.
He said people should work from home and avoid pubs, club, theatres where possible, as part of a range of stringent new measures.
Anyone in the same house as someone who has a cough or fever will also have to isolate for 14 days, he said.
All elderly and vulnerable people must also begin self-isolating within days.
The first person in Wales to die with Covid-19 brings the UK total to 36.
In the first of a series of daily briefings on the virus, which causes the Covid-19 disease, Mr Johnson said “drastic action” was needed as the UK approaches “the fast growth part of the upward curve” in the number of cases.
Mr Johnson said that by next weekend, those with the most serious health conditions must be “largely shielded from social contact for around 12 weeks”.
Sir Patrick Vallance, the UK’s chief scientific adviser, said other measures may be necessary – including school closures – at some point.
“Those things need to be done at the right time,” he said.
The UK government’s chief medical adviser, Prof Chris Whitty, said the chance of dying with the virus “for any individual person” was “very low”.