• Video report by ITV News Science Editor Tom Clarke

Eligibility for coronavirus testing has been expanded to include "all essential workers" in England and those in their household, the health secretary has said.

Speaking at the government's daily coronavirus press conference, Matt Hancock said the government was making it "easier, faster, and simpler for any essential worker in England, who needs a test, to get a test".

He said from Thursday, employers of essential workers can book tests for any of their staff online on gov.uk, while from Friday, any essential worker will be able to book their own.

He said the "ultimate goal is that everyone who could benefit from a test, gets a test".

He said he was able to expand eligibility due to capacity for testing increasing "so substantially" from 40,000 yesterday to 51,000 today.

The government is still way off its target of 100,000 tests by the end of April - a task the health secretary said was "challenging".

Despite needing to increase capacity by 50,000 tests within a week, Mr Hancock said the government was “ahead of our plans” on capacity.

National Testing Coordinator Professor John Newton, speaking alongside Mr Hancock, said the UK is "on track to reach a hundred tests a day as planned".

"In fact", he added, "We're somewhat ahead of where we thought'd we'd be at this stage."

He pointed to 48 drive through testing facilities which will be open by the end of the month, with mobile test units attached to each site being able to visit care homes and test any residents and staff.

Separately, Amazon is being used to deliver tests to the homes of people who need them.

Mr Hancock said the UK planned to use a "rigorous programme of test, track and trace" to learn where coronavirus is and where it had been in the country.

“These are critical pieces of information to inform our battle against this novel virus,” he said.

He said initially, 25,000 people would take part in the contact tracing programme, with plans to expand it to 300,000 over the next 12 months.

He said letters asking people to participate in the programme were "arriving on doorsteps" already and he urged anyone who receives one to "take part in this vital research for your country".

“The early signs from today are that there is huge enthusiasm from those who have received letters taking part in this survey,” he said.

Mr Hancock said the infrastructure was being put in place so that contact tracing could be rolled out on a larger scale.

“As we look ahead, this is critical to keep the virus under control,” he said.

Mr Hancock said a new NHS contact-tracing app was undergoing testing.

Anyone who became unwell with coronavirus symptoms would be able to inform the NHS through the app, which would then inform other users they had had significant contact with.

Chief scientific adviser Sir Patrick Vallance stressed the importance of coronavirus patients enrolling into clinical trials to find out which medicines may help treat the disease.

He told the press conference: “I would just urge as we enter the phase where the plateau has been reached, slight decrease we can see, we continue to make sure that we enrol patients in clinical trials so we get the answers to the critical questions about which medicines may work.”

Sir Patrick also said he expected the plateauing of the death figures to “continue for another couple of weeks and we will then see a faster decline thereafter”.

Mr Hancock said: "We need to bring the incidents of new infection right down" before lockdown restrictions can be relaxed.

When asked about government transparency on the lockdown exit strategy, Mr Hancock said he understands "the thirst for knowledge" but declined to reveal what the plan might be.

"Of course, monitoring what is happening and making sure that we move at the right time is absolutely critical," he said.

To lift the measures too soon and to risk a second peak, he said, "would be a mistake - it would undo all of the hard work that’s been done".

"It would be both bad for our nations health and bad for our nations economy".

Earlier, the government received advice from the Scientific Advisory Group for Emergencies (Sage) on whether it would be useful to make the wearing of face masks compulsory in public.

Sir Patrick Vallance said the information available on the effectiveness of masks remained “weak”.

He said: “The evidence on face masks has always been quite variable, quite weak and difficult to know.

“There aren’t any real trials on it.

“We have undertaken a review, we will give our advice to ministers and they will make decisions around what to do around that.”

The press conference follows the latest figures from the Department for Health, which showed the overall death toll from the virus in hospitals to 18,738 - an increase of 616 from the previous day.