Britain is to restart tracing the contacts of people who have had coronavirus symptoms, the health secretary has said, reverting to a policy recommended by the World Health Organization but abandoned by the UK as the numbers of cases and deaths began to rise in early March.

Matt Hancock told the health select committee on Friday that police, fire, prison and Department of Work and Pensions staff would now be eligible for coronavirus testing, amid growing concerns that the country will miss its target of 100,000 coronavirus tests a day by the end of April.

A former director of the WHO told the hearing that contact tracing, testing and isolation could have continued for longer across the UK and would have enabled the government to lock down London while leaving other areas of the country with fewer restrictions.

Prof Anthony Costello, the head of the Institute for Global Health at University College London, pointed out that Yorkshire had fewer than 10 cases identified in 300,000-400,000 people around the time that contact tracing and community testing were halted and, as such, could have avoided a complete lockdown.

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Critics have said the UK was too quick to stop the universal testing of people with symptoms, and following up with their recent contacts, who could then be asked to self-isolate.

The WHO has repeatedly urged governments to pursue the strategy, which has been key to stopping epidemics around the world in modern times. “Tracing every contact must be the backbone of the response in every country,” the director general, Tedros Adhanom Ghebreyesus, said in March.

Other countries such as Singapore and South Korea, have successfully used it to contain their outbreaks, while Germany, which has a far lower case and death rate than the UK, has also worked hard on contact tracing.

But on 12 March, Boris Johnson announced that the UK epidemic could no longer be contained that way. Testing was then restricted to those who were admitted to hospital, and contact tracing was stopped. The numbers of cases and deaths in the UK soared and a lockdown was imposed on 23 March.

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Hancock said on Friday that the government would rebuild the teams of contact tracers as part of a renewed testing and tracing strategy.

His predecessor Jeremy Hunt, who is chair of the health select committee of MPs where Hancock was giving evidence, pointed out that South Korea had up to 1,000 contact tracers. “Public Health England had 290 people doing this at their peak, but they now say it has been wound down. Are we going to build it up again?” he asked.

Hancock said yes and that contact tracing teams would be part of a strategy that included the use of a phone app to identify recent contacts and warn them that they might need to self-isolate.

“That brings together teams from NHSX, who are leading on the app, with a huge array of partners that are working within that, [and] from Public Health England, who are the experts on what we refer to as external contact tracing,” he told the committee.

“The app is itself a contact tracing app, that is the point of it, to be able to assist individuals to do contact tracing themselves by notifying people who they have been in close contact with when they have downloaded the app. And then of course link that to testing, so people can get the tests.”

Testing, tracing and isolating the contacts of people with Covid-19 is expected to be a key part of the exit strategy for every country that has imposed a lockdown, as a way to keep control of the epidemic. Asked if he accepted that comprehensive contact tracing would need to be in place to lift the UK lockdown, Hancock said the government was working “incredibly hard” on the WHO-recommended strategy.

“We do need to have comprehensive test, track and trace in place as soon as possible,” he said. “And we need to get the technology right, we need to have the people, and we’re building that resource, and obviously we need to have the testing and we’re ramping that up as well. So we do need to have all three of those in place and we’re working incredibly hard to make sure that we are.”

Earlier, Costello said the nationwide lockdown could have been avoided, with only the capital facing severe restrictions.

He said the example of 10 identified cases in 300,000-400,000 people in Yorkshire meant that in such areas people could have continued to go about their lives and work while maintaining physical distancing.

Hancock admitted the government had discussed it. “We did consider having a London-specific lockdown and decided it was better to do it across the country as a whole.”

This was because people would be travelling between London and other parts of the UK and, he said, it was striking how the country had come together to observe the restrictions. “To separate one part of the country from another has downsides in terms of the national unity we have seen in the response.”

Johnson’s spokesman later declined to say anything beyond Hancock’s words on the possible scale of testing needed to implement a full regime of contact tracing.

He also declined to comment on why the UK did not isolate or trace people arriving in the country from overseas, while adding that this policy could potentially change in the future: “We’ve always said that we don’t rule out taking steps if that’s what the science advises.”