The UK could be forced under a full lockdown within hours if the public does not heed pleas to stay two metres apart outdoors.
The Prime Minister responded to scenes of huge crowds flocking to Britain's parks, beaches, and holiday spots on Mother's Day yesterday.
Boris Johnson said there was no ban on going outdoors for fresh air - yet - as he told Brits to stay home as much as possible.
But the PM warned in yesterday's press conference the Government is ready to impose tougher restrictions to curb the spread of the coronavirus if people do not follow the guidance on social distancing and stay two metres apart.
He warned: "I don't think you need to use your imagination very much to see where we might have to go, and we will think about this very very actively in the next 24 hours."
His warning came as the latest official figures showed the number of people across the UK who have died after testing positive for Covid-19 has risen by 48 to 281 - including an 18-year-old, thought to be the youngest victim so far.
The PM insisted that he did not want to close down access to parks and playgrounds because of the benefits of the outdoors to people's mental and physical wellbeing.
But amid stunning sunny weather across the UK on Sunday, the authorities were not impressed by the scenes of crowds flocking to parks, beaches and beauty spots.
Mr Johnson warned Brits could soon face the "tougher" measures adopted in other European countries like Italy and France if people did not behave responsibly.
Mr Johnson has faced criticism for not acting faster to slow the spread of the virus.
He also raised questions when he appeared to contradict himself by telling Brits not to visit their mums on Mother's Day while saying he hoped to see his own mum.
He later said he would be calling her.
The government has already shut down pubs, clubs and restaurants across England, Scotland, Wales and Northern Ireland.
But authorities were dismayed by the scenes over the weekend as pictures and footage flooded in from around the country.
Huge lines crowded to climb the Snowdon peak in Wales, and in London, large groups and cyclists packed out popular parks.
Several local authorities, including Hammersmith and Fulham Council, took the step of closing all their parks in response to the crowds.
And the Royal Parks issued a warning that while its parks remain open for now, it could be forced to close them, as it announced it was shutting all remaining take-away cafes and kiosks: "with immediate effect as people are not adhering to social distancing guidelines."
Mr Johnson said Government had always followed the scientific advice when it came to access to open spaces - but warned people needed to observe the guidance that they should not gather in groups and remain two metres apart.
"What they have always said so far is that the health benefits for the whole of society of keeping the parks and playgrounds open if we possibly can outweigh the epidemiological value of closing them," he said.
"But of course looking at the way people behave and the way they are responding, we keep that under constant, constant review.
"If people can't make use of parks and playgrounds responsibly, if they can't do it in a way that observes the two-metre rule then of course we are going to have to look at further measures.
"The general principle should be that we should all as far as we possibly can stay home, protect our NHS and thereby save lives."
He added: "I don't think you need to use your imagination very much to see where we might have to go, and we will think about this very very actively in the next 24 hours."
The Government later issued updated guidance on essential travel, saying it did not include visits to "second homes, camp sites, caravan parks or similar, whether for isolation purposes or holidays".
Earlier, in a message to the nation, the Prime Minister warned Britain was only "two or three" weeks behind Italy where the death toll has already risen above 5,000 making it the worst outbreak anywhere in the world.
In the UK, the death toll stands at 281, with 5,683 confirmed cases.
However experts have warned the number of cases is likely much higher, as many Brits and NHS staff say not everyone with Covid-19 symptoms is being tested.
He said that unless the UK could control the spread of the virus through social distancing, the NHS would be "overwhelmed" in the same way that the Italian healthcare system had been.
However the deputy chief medical officer for England, Dr Jenny Harries, cautioned against comparing the UK's figures too closely with other countries'.
"As you go through a sharp rise in numbers, which we will see going forward, you have to be careful to be comparing too precisely," she said.
"We will look back in due course, sadly, and see the true number of people who have died from coronavirus."
Health Secretary Matt Hancock has since warned the Government "may have to take further action" if people continue to flout social distancing rules of staying two metres away from each other.
He told Sky News: "This is not the sort of thing that anybody would want to do but, of course, it is the sort of thing we might have to do in order to protect life.
"If you do go out, you must not get closer than two metres from someone who isn't in your household.
"It is a really simple rule and incredibly important, because to protect life and the NHS we need to stop the spread of this virus and the virus spreads by people coming into close contact with each other."
Elsewhere other political figures adopted a notably more strident tone than the Prime Minister as they warned people not to leave their homes other than for essential business.
London Mayor Sadiq Khan, whose city is at the centre of the epidemic in Britain, has ordered the city to stay off its public transport networks, including the London Underground, unless it is for essential travel.
Mr Khan said people must obey the guidance or others would die.
"This isn't advice, as far as I'm concerned. These are instructions and these are rules that we should all obey to stop people dying," he told BBC1's The Andrew Marr Show.
Scotland's First Minister Nicola Sturgeon announced she was closing the ferries to the Scottish islands to all non-essential travellers after the military was brought in to transport a patient to the mainland.
She said people should not go shopping "except for essentials like food and medicine" and she condemned people "flocking" to Scotland's remote communities for adding to the pressure on local health services.
"Beaches should not be busy, parks should not be full," she said.
Meanwhile the NHS is writing letters to 1.5 million determined to be Britain's most vulnerable people.
Mr Johnson said the "shielding" of the 1.5 million, who will be required to stay at home for 12 weeks - would do "more than any other single measure to save life".
They include people with severe respiratory conditions and those suffering from some cancers such as those of the blood or marrow.
Communities Secretary Robert Jenrick said that, with the help of military planners, the Government was creating a network of local "hubs" to ensure those without family or friends to support them received their medicines and other vital supplies.
"Nobody needs to worry about getting the food and essential items that they will need," he said.
