Government plans to purchase antibody tests to detect if someone has had coronavirus could be a "gamechanger" in the UK's response to Covid-19, the Prime Minister has said.
Boris Johnson said "hundreds of thousands" of kits could be bought if the tests prove effective - though negotiations are currently still ongoing.
It comes as the number of people tested positive for coronavirus in the UK rose to 3,269.
The death toll from the virus in the UK stands at 144.
According to Public Health England (PHE), understanding who has previously had the virus will allow authorities to better refine their estimates of how many people in the population will be affected and the rate of spread.
Speaking about testing on Thursday, Mr Johnson said: "To give you an idea of what is coming down the track, we're in negotiations today to buy a so-called antibody test".
The Prime Minister said it would be "as simple as a pregnancy test which can tell whether you have had the disease".
He added: "It's early days, but if it works as its proponents claim, then we will buy literally hundreds of thousands of these kits as soon as practicable.
"Because obviously it has the potential to be a total gamechanger."
The human body produces antibodies when it fights an infection, so by measuring antibodies in the blood, doctors can detect whether someone has had the infection previously.
Public Health England (PHE) said it was currently developing a blood test for antibodies by analysing blood samples from people who have recovered from coronavirus infection.
The test and its criteria are still in development, but PHE expects it to play a key role in its surveillance of infections - beginning with 1,500 tests a week.
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In the Government's daily coronavirus update on Thursday, Chief Medical Officer for England Professor Chris Whitty said: "On the antibody tests we should be clear that although we're confident there will be antibody tests, we're not absolutely confident yet about whether the ones that are currently on the market are the right ones and that needs to be tested out.
He added that that was "an absolute priority" for PHE.
Prof Whitty added that once the tests are available, "we'll be able to say to somebody, you've had this virus, youre not likely to get it again at least in the immediate term, and now we can be confident you can return to work and now we can be confident you don/t need to be taking some of the precautions you’ve been taking to date."
Prof Whitty said it would be "a while" before the tests where available in large numbers, but said they would be "more useful" further along in the outbreak.
Boris Johnson added that authorities were also "massively increasing" testing on people who may currently have coronavirus.
He said daily testing is going "from 5,000 a day, to 10,000 to 25,000 and then up at 250,000".
It follows criticism over the Government's approach to testing, with calls for NHS frontline workers to have better access to tests.
As of 9am on Thursday 64,621 people in the UK had been tested for coronavirus - with 3,269 confirmed cases.
Mr Johnson said UK scientists were expect to start trials for the first coronavirus vaccine with a month.
Chief scientific adviser Sir Patrick Vallance said the first trials patient was enrolled on Thursday, with a start date of mid-April expected.
While scientists in the US have already begun experimental dosing of a potential vaccine.