Boris Johnson is still showing signs of coronavirus, Downing Street has said, as it confirmed the Government is working with nine potential suppliers over a new Covid-19 antibody test.
The Prime Minister's seven days of self-isolation end on Friday but it is unclear whether he plans to leave the Downing Street flat where he has been staying.
It comes as 2,921 people were confirmed to have died in hospital after testing positive for coronavirus in the UK as of 5pm on Wednesday.
The youngest person who died without underlying health conditions was aged 25.
The total is up by 569 from 2,352 the day before and is the biggest day-on-day increase so far, just above the 563 reported the day before.
The Prime Minister's official spokesman told a Westminster briefing: "The Prime Minister continues to have mild symptoms, but he does still have symptoms."
Asked if he would be leaving self-isolation on Friday, the spokesman said: "We're following the guidelines from Public Health England (PHE) and from the chief medical officer which state that you need to self-isolate for a period of seven days, so no change in that."
Number 10 said work was ongoing with nine potential suppliers on developing an antibody test which would show whether people have had the virus.
Such a test would enable people to get back to work quickly and some experts say this type of testing is the quickest way out of the current lockdown.
The Prime Minister's spokesman said: "We are working as quickly as we can on that and as soon as a test is approved then we will announce it publicly."
He said the Government had previously been offered tests that had not met the required levels of accuracy "and therefore would not have been safe to use".
It was also suggested that immunity certificates to identify people who have had coronavirus are being considered by the Government.
The Prime Minister's spokesman said this had been discussed in other countries and the UK was watching what happened closely.
It comes as Health Secretary Matt Hancock, who is now out of self-isolation, is expected to announce new details of increased test capacity across the UK.
This follows fierce criticism of the policy on testing, with the UK only hitting a target of 10,000 tests per day in the last few days.
Meanwhile, comedian Eddie Large, best known for being half of British comedy duo Little And Large, has died after contracting coronavirus in hospital.
The 78-year-old, whose real name was Edward Mcginnis, had been suffering with heart problems, his son Ryan said in a Facebook post.
Number 10 said Mr Johnson will look to find a way to participate in the "Clap for our carers" celebration on Thursday evening despite remaining in self-isolation.
"The PM definitely wants to find a way of expressing his continued appreciation for the fantastic efforts that NHS staff are making and the PM would urge as many people as possible to take part tonight," his spokesman said.
The latest testing figures show 10,657 tests were carried out on Tuesday, the Department of Health tweeted. Some 2,800 NHS staff have now been tested for coronavirus at drive-in facilities.
On Wednesday, Mr Johnson took to Twitter to stress the importance of testing, saying it is how "we will unlock the coronavirus puzzle".
On Thursday, Professor Paul Cosford, emeritus medical director of Public Health England (PHE), admitted "everybody involved is frustrated" by the low number of tests being carried out.
He pointed to ongoing capacity issues and said the "core priority" until now has been testing hospital patients with suspected Covid-19.
PHE's focus has been on NHS testing laboratories, while other work is now being led by the Office for Life Sciences to collaborate with universities and non-PHE labs, he said.
Prof Cosford said five drive-through NHS staff testing hubs are up and running, with "another four to come on stream this week".
Testing will hit 15,000 per day "imminently", he said, adding: "It will be 25,000 by the middle of April."
Work being led by the Office for Life Sciences "to look at a much broader set of universities, industry and other laboratories" will "give us another 100,000 or more tests per day", Prof Cosford continued.
"We've certainly not refused any help from any laboratories and we've talked to many about what might be possible, he said, but added: "We need to be very careful to make sure that the tests we use are tests that work."
Asked why it is taking so long to increase testing, Prof Cosford told Good Morning Britain: "This is an incredibly complex operation to put in place in a very short period of time."
He added that there is "24/7 work" going on to overcome "a whole range of issues" in ensuring testing is rolled out properly.
On BBC Radio 4's Today programme, Prof Cosford added that "everybody involved is frustrated" about not reaching the required testing output.
He said an antibody test is being evaluated "as we speak to see whether it will deliver what we need it to deliver".
Shadow health secretary Jonathan Ashworth tweeted on Thursday there was a need for national testing strategy.
He added: "Community testing & tracing is way out of lockdown cycles until vaccine found."