Police will be able to issue "unlimited fines" to those who do not comply with new rules on social distancing, the health secretary has told the House of Commons.
Matt Hancock told MPs that gatherings of more than two people in public are now banned and said police would be enforcing the new rule with fines.
"These measures are not advice, they are rules and will be enforced, including by the police with fines starting at £30 up to unlimited fines for non compliance," he said.
He added: "Home is now the front line, and in this national effort, working together, we can defeat this disease.
"Everyone has a part to play."
Boris Johnson's statement was followed up with a Government text message, sent out on Tuesday morning, urging people to stay at home and providing details of the new coronavirus lockdown rules is being pushed out across the UK.
Some mobile phone users have reported already receiving the text, with more expected to see it throughout the day.
The lockdown will stay in place for at least three weeks.
Speaking from Downing Street in an historic address to the nation, the Prime Minister declared a "national emergency" and said it was vital people listened to the advice in order to save lives.
He said that people would only be allowed to leave home for:
shopping for basic necessities, as infrequently as possible
one form of exercise a day - for example a run, walk, or cycle - alone or with members of your household;
any medical need, to provide care or to help a vulnerable person;
and travelling to and from work, but only where this is absolutely necessary and cannot be done from home.
Click above to watch the Prime Minister's address to the nation in full
Mr Johnson added: "If you don’t follow the rules the police will have the powers to enforce them, including through fines and dispersing gatherings."
All shops selling non-essential items will be closed, along with libraries, playgrounds, outdoor gyms and placed of worship.
Weddings, baptisms and other ceremonies will be halted, although people will still be allowed to attend funerals.
Reiterating the Prime Minister's message, a text message sent on Tuesday read: “GOV.UK ALERT CORONAVIRUS. New rules in force now: you must stay at home. More info and exemptions at gov.uk/coronavirus Stay at home. Protect the NHS. Save lives.”
Because the UK does not have an emergency alert system, network operators are sending out the text on behalf of the Government, resulting in it arriving at different times.
South Korea’s success in slowing the spread of Covid-19 has been at least partly attributed to an aggressive text messaging system deployed by the country, where alerts were sent out detailing the movements of people who had tested positive.
The alerts come as cyber security experts encourage people to be aware of potentially malicious texts and emails, where criminals pose as official agencies to try and elicit personal information from users.
The UK’s National Cyber Security Centre (NCSC) has warned of increased activity of this type, highlighting incidents where clicking on links in bogus emails claiming to have important updates has led to devices being infected.
These “phishing” attempts can lead to victims being left out of pocket or stripped of sensitive data.
Speaking on Monday night in a televised address to the nation, Mr Johnson said: "No Prime Minister wants to enact measures like this.
"I know the damage that this disruption is doing and will do to people’s lives, to their businesses and to their jobs.
"And that’s why we have produced a huge and unprecedented programme of support both for workers and for business.
"And I can assure you that we will keep these restrictions under constant review. We will look again in three weeks, and relax them if the evidence shows we are able to.
"But at present there are just no easy options.
"The way ahead is hard, and it is still true that many lives will sadly be lost.
"And yet it is also true that there is a clear way through."
He thanked the efforts made by NHS staff, saying: "The people of this country will rise to that challenge.
"And we will come through it stronger than ever.
"We will beat the coronavirus and we will beat it together.
"And therefore I urge you at this moment of national emergency to stay at home, protect our NHS and save lives."
Coronavirus: Everything you need to know
Labour leader Jeremy Corbyn tweeted: "The Prime Minister is right to call for people to stay at home, protect our NHS and save lives."
Mr Corbyn added: "This is the right response to the coronavirus pandemic, and one we have been calling for.
"There now needs to be clear guidance to employers and workers about which workplaces should close - and the Government must close the loopholes to give security to all workers, including the self-employed, as well as renters and mortgage holders.
"We welcome these moves and will be working to ensure everybody has the protection and security they need."