Health Secretary Matt Hancock said the NHS in England has the capacity to conduct 161,000 tests a day, telling the Downing Street briefing: "Because of that increased capacity, I can announce that we're expanding eligibility yet further.

"From tomorrow, we're expanding eligibility for testing to include the under-5s so that every single person who has symptoms of coronavirus can get a test, no matter their age."

Mr Hancock appeared in front of a new lectern slogan, which read "NHS Test and Trace", as he confirmed further details on the UK's approach.

He said the test and trace scheme is the start of replacing the national lockdown with "individual isolation" for those who have been in contact with the coronavirus, along with "local action where it's necessary to respond to a flare up".

Mr Hancock said it would enable them to "hunt down the virus" to find out who has been infected, telling the Downing Street briefing: "We all have our part to play.

"This is a national effort and we all have a role. If you have symptoms, you must isolate immediately and get yourself a test."

Mr Hancock said contact tracing would enable an NHS clinician and the person with the virus to work together "like detectives" to identify the possible movements of the virus.

He went on: "We must all follow the NHS test and trace instructions as this is how we control the virus, protect the NHS and save lives."

Mr Hancock said: "If you get symptoms, isolate immediately and get a test.

"If you are contacted by NHS test and trace instructing you to isolate, you must.

"It is your civic duty, so you avoid unknowingly spreading the virus and you help to break the chain of transmission."

The Health Secretary added: "This will be voluntary at first because we trust everyone to do the right thing.

"But, we can quickly make it mandatory if that is what it takes.

"Because, if we don't collectively make this work, then the only way forward is to keep the lockdown."

The executive chairwoman of NHS Test and Trace Baroness Dido Harding outlined how the new system would work.

Baroness Harding said: "I want you to feel safe and confident to play an active part in NHS Test and Trace, for you, your loved ones and our country. We do need you to follow the following three steps:

"Step one - if you have one or more of the symptoms of coronavirus, a fever, a new, continuous cough or loss of your sense of taste or smell, you must immediately self-isolate.

"Step two - you should then book a test on the site, or if you don't have internet access, by dialling 119. Do not leave home for any other reason.

"If you test positive, you will then be contacted by the NHS Test and Trace service within 24 hours. All contact tracers have been undergoing training and induction before the beginning of this week and before they start work."

Mr Hancock said: "This system will start tomorrow morning at 9am.

"And the first people who will be contacted will be the people who received a positive result today.

"This is a very distinct change on our approach."

Baroness Harding added that compliance with the new test and trace system is "vital" to carefully lift the remaining lockdown measures.

She said: "Step three - NHS Test and Trace will help you establish who you've been in close contact with and so who you might have infected and will gather their contact details.

"This could include members of your household, or someone you've been in two metres of for more than 15 minutes."

Baroness Harding added: "NHS Test and Trace in turn will get in touch with those contacts.

"So if you've been exposed to an infected person, they will be in contact with you. You will then be instructed by the NHS to self-isolate for 14 days, even if you don't have symptoms or you feel perfectly well. You need to follow these instructions.

"This individual and collective effort is vital if we're to keep the rate of infection down and carefully lift the lockdown."

Asked about easing the lockdown for those aged under 45 given the lower risk of death from Covid-19, Mr Hancock said the evidence shows those in the age group are just as likely to get the virus and the impact on spreading the disease is "just as great".

Professor Jonathan Van-Tam, deputy chief medical officer, also told the Downing Street briefing: "The infection rates are not any lower in under-45-year-olds.

"Their propensity to transmit the infection is probably greater than those of an older age group simply based on the number of social contacts and social networking that they have the potential to do in an unrestricted way.

"The whole game about beating Covid-19 is, for now, until we get a vaccine, until we get effective antiviral drugs, is to reduce contact between people and particularly reduce contacts between households to a level that is safe."