The pledge came on the day the UK recorded more than 500 daily deaths for a second consecutive day.

Johnson was speaking from self-isolation following criticism of his government's failure to provide widespread screening, particularly for frontline healthcare workers.

The health ministry announced a record 569 deaths from the virus in Britain in the 24 hours up to 1600 GMT on Wednesday - the largest single-day rise yet.

It followed 563 deaths over the previous corresponding period.

The ministry has recorded a further 4,244 confirmed cases in hospitals, taking the number of positive tests to 33,718 as of 0800 GMT on Thursday.

Johnson has been in self-isolation at his Downing Street official residence since announcing on March 27 that he had caught the virus.

"The PM continues to have mild symptoms," his official spokesman told reporters on Thursday.

"We will follow the best medical and scientific advice," he added when asked if the British leader would end his quarantine on Friday, after the recommended seven days of isolation.

In an online video message posted on Wednesday evening, Johnson, whose initial light-touch approach to the outbreak came under scrutiny, said testing was the "way through" the crisis.

"We're also massively increasing testing. As I have said for weeks and weeks this (testing) is the way through," he said.

"This is how we will unlock the coronavirus puzzle, this is how we will defeat it in the end."

Johnson is facing criticism even in normally supportive media outlets after officials revealed that just 2,000 out of some 500,000 staff in the state-run National Health Service (NHS) had been tested.

Health minister Matt Hancock responded on Thursday by saying that efforts had been ramped up, with 5,000 NHS staff now tested.

The minister, who was giving his first news conference since leaving isolation after testing positive himself, added that the government was "determined" to scale up tests across the board in the coming weeks.

"I'm now stating the goal of 100,000 tests per day by the end of this month. That's the goal and I'm determined that we will get there."

Hancock blamed global demand for swabs and reagents for the lack of tests, and said that some they had bought were faulty.

In order to meet the demand, the government said it would work with private firms such as Amazon and chemist Boots, and that three new "mega labs" would soon be online.

Testing for the general public has also been condemned as not being widespread enough and is currently largely limited to hospital admissions of the most serious Covid-19 patients.

On Tuesday, 10,000 hospital patients and NHS staff were tested in England, well below the daily target of 25,000 and the 70,000 a day achieved in Germany, which has been used as a comparison.

Paul Nurse, chief executive of biomedical research centre the Francis Crick Institute, told the BBC on Thursday that the government should summon "the Dunkirk spirit" and let 'small ship' labs start screening for the killer disease.

So far, Public Health England (PHE), the body tasked with testing, has insisted all screening should be carried out centrally.

PHE medical director Professor Paul Cosford defended his organisation's work.

"At the very outset we identified this, we got the tests in place, we designed the tests in our laboratories. We have played our part," he told BBC radio.

Britain is currently in the second week of a three-week lockdown, with non-essential shops shut and the public asked to stay at home to try to limit the spread of coronavirus.

The government has promised an enormous package of support for businesses and employees hit by the measures.

New government figures show 950,000 people applied for state welfare support known as universal credit in the last two weeks. It is available to the unemployed and those on low incomes.

With economic headwinds gathering pace, national carrier British Airways is also to temporarily lay off 28,000 staff, the union representing its workers announced on Thursday.