It comes ahead of schedule but weeks after similar test-and-trace schemes were introduced in Germany and France.
England's new COVID-19 test-and-trace system launches on Thursday.
It is being launched several days ahead of schedule but weeks behind similar ones implemented in Germany and France.
COVID-19 has hit the UK harder than any country in Europe.
UK Prime Minister Boris Johnson had earlier promised that the service would be "world-beating" and go live on June 1.
What does it involve?
From today, anyone who tests positive to COVID-19 in England will be contacted by one of the country's 25,000 tracers.
According to the government, the dedicated contact-tracing staff will have the capacity to trace the contacts of 10,000 people who test positive per day — the UK's total number of infections has risen daily by between 1,600 and 4,000 over the last week.
These contact tracers will urge infected people to self-isolate for at least seven days or until symptoms have passed.
They will also demand the person identifies any recent contact they've had and alert those most at risk. These include household members, people they've been in direct contact with or within 2 metres for more than 15 minutes.
These people will then, in turn, be asked to self-isolate at home for 14 days even if they do not have any symptoms. If they develop symptoms, they will be able to book their own test.
'New way of life'
The UK's health minister Matt Hancock said on Wednesday during the government's daily COVID-19 briefing from Downing Street that "testing and tracing must become a new way of life".
He stressed that people told to self-isolate by contact tracers must do so.
"It is your civic duty so you avoid unknowingly spreading the virus and you help break the chain of transmission," he said, warning that while it will for now be done on a voluntary basis, the authorities "can quickly make it mandatory if that's what it takes".
"If we don't collectively make this work then the only way forward if to keep the lockdown," he added.
Northern Ireland already has a system in place while authorities in Wales are working on bringing their own systems in as quickly as possible.
Scotland is also set to launch its own system on Thursday.
'Particular lessons' to draw from South Korea and Germany
The UK government has been harshly criticised for its decision to abandon mass testing and tracing in mid-March as the number of cases and fatalities soared.
The country is the second hardest-hit in the world after the US with more than 37,500 deaths, according to a tally kept by the Johns Hopkins University.
The UK ramped up testing to more than 100,000 per day by the end of April. Johnson also promised the implementation of a "world-beating" test and trace system.
The system now in a place closely resembles the one deployed in Germany where local health authorities made use of contact tracers from as early as mid-March.
Professor Dame Angela McLean, Chief Scientific Adviser to the UK's Ministry of Defence, told reporters last week that the country could draw "particular lessons" from South Korea and Germany.
She said the Asian country "made inspiring use of all kind of contact tracing in order to control infection" while Germany has shown the importance of testing.
Fewer than 8,400 people have lost their lives to the pandemic in Germany, according to the country's Robert Koch Institute for Infectious Diseases, while 179,364 confirmed infections have bee recorded.
In France, each of the country's 101 departements (counties) were instructed to have dedicated contact tracing staff by the time the first lockdown restrictions were lifted on May 11.