A massive system to find people who come into close contact with those infected with coronavirus will start in England on Thursday, Boris Johnson has said.
The prime minister said it "will change people's lives".
The aim of the test and trace system is to move from lockdown for all towards more targeted measures.
However, scientists have warned it is not a "magic bullet" and may prevent between 5% and 15% of infections.
What will I have to do now?
As is currently the case, anyone who develops symptoms of coronavirus - a persistent cough, fever or a sudden loss of taste or sense of smell - will have to isolate for seven days and the rest of their household for 14 days.
The difference is that from Thursday, everyone with symptoms should ask for a test online or call to arrange a test by calling 119.
If the test comes back negative, everyone in your household can go back to normal.
But if the test comes back positive, the NHS Test and Trace team will get in touch - via text, email or phone call - to discuss whom you have come into close contact with.
Any of those contacts deemed at risk of catching the virus will be instructed by the NHS to go into isolation for 14 days, whether they are sick or not.
They will be tested only if they develop symptoms. The rest of their household does not have to isolate, unless someone becomes ill.
Those isolating will be eligible for statutory sick pay.
Health Secretary Matt Hancock told the daily Downing Street briefing this "must become a new way of life" and will require a "national effort" - otherwise lockdown would have to continue.
"It is your civic duty, so you avoid unknowingly spreading the virus and you help to break the chain of transmission," he said.
He also confirmed that, from Thursday, anyone with symptoms - including the under-5s - can be tested for coronavirus.
Earlier, Mr Johnson told MPs on the Commons Liaison Committee that the government "will consider bringing in financial sanctions" if people do not self-isolate.
What is a close contact?
Only people who have been "close" to an infected person will be contacted by NHS Test and Trace.
It will not be the case that one teacher testing positive would lead to the whole school going into isolation, for example.
Close contacts are:
- people you spend 15 minutes or more with at a distance of less than 2m
- people you have direct contact with - such as sexual partners, household members or people with whom you have had face-to-face conversations at a distance of less than 1m
The contact must have taken place between two days before and up to seven days after symptoms appeared.
What is the point?
At the moment coronavirus is being suppressed by lockdown restrictions which are applied across the whole of society.
The ambition is to pivot from lockdown for the many to isolation for the few. How far that can go will depend on the effectiveness of the scheme.
Baroness Dido Harding, the chairwoman of NHS Test and Trace, said: "[It] is designed to enable the vast majority to get on with their lives in a much more normal way... trading national lockdown for individual isolation."
Test and trace will also give far more detail on how and where the virus is spreading. Mr Hancock said this information could lead to local lockdowns to tackle flare-ups in towns, schools or workplaces.
Will it work?
A report published earlier by the Royal Society said the success of the scheme would depend on how quickly contacts can be found and whether the public buys in.
It concluded that between 5% and 15% of infections could be prevented. But the 15% figure depends on finding contacts within three days and 80% of people reporting their symptoms or going into isolation when asked.
Prof Anne Johnson, one of the report's authors, said: "It is an important part in bringing the pandemic under control, but we're very clear it is not a magic bullet."
A lot to get used to
This is a big ask - there will be a lot for people to get used to.
First, if you test positive for the virus you will be expected to pass on contact details of people you have recently met. Some may be concerned about that though NHS leaders emphasise that confidentiality and data security will be rigorously observed.
On the other hand you may get a call from a contact tracer and advised you are at risk because of a recent encounter with someone with the virus. And even if you are fit and well you will still be told to self-isolate for two weeks.
Health Secretary Matt Hancock has called on people to do their civic duty. The big question is whether sufficient numbers will respond positively to that.
Sanctions have not been ruled out if they don't. There's a lot riding on policies like this to be adopted in every part of the UK, not least as a vehicle to help each nation out of lockdown.
Instead, social distancing, hand hygiene and isolating everyone who gets ill will have a far greater effect, the report said.
By comparison, isolating cases and quarantining members of their household is thought to cut infections by 50%.
And scientists, including those advising the government, have warned that the row over the PM's chief adviser Dominic Cummings will make it harder to get public support such measures.
Can the system cope?
The government says it has 25,000 contact tracers ready to go and that it has plenty of testing capacity.
The contacts of 2,000 people who have just tested positive for coronavirus will be chased on Thursday.
However, the real pressure will be on the speed of testing.
Prof John Newton, the national coordinator of Test and Trace said: "In order to be successful it requires rapid testing."
Mr Johnson said on Wednesday there was now a target to get test results in less than 24 hours.
There is a particular problem with home testing kits, which have to be posted out. Prof Newton said it would be "very difficult" to get those results in less than 48 hours.
The NHS contact tracing app is not ready to roll out and was described by Mr Hancock as a "complement". He denied technical issues were delaying its roll out.
In other developments:
- A further 412 coronavirus deaths were recorded in all settings across the UK, taking the confirmed death toll to 37,460
- The prime minister said he has asked scientists to review the 2m social distancing rule as the virus is suppressed in order to help public transport and the hospitality sector
- Mr Johnson has ruled out an inquiry into Mr Cummings' conduct during lockdown, insisting it was time to "move on" from the row
- Local lockdowns are not being considered in Wales to maintain a "clear message" about coronavirus, the Welsh Government has said
Follow James on Twitter