Britain is under pressure to increase the number of coronavirus tests it carries out, as it emerged that public health chiefs have never once hit their initial 10,000-a-day target.
The UK is now seriously lagging behind other nations in monitoring the spread of the virus, and testing NHS workers to see if they are infected.
Experts have warned that in some hospital wards, up to one third of medics are now self-isolating at home, often because family or housemates have shown signs of the virus. If they could be tested, they could get back to work, say health analysts.
On March 11, NHS England said it would be ramping up testing to 10,000 a day, and called on local hospital laboratories to help with the effort. A week later Boris Johnson vowed that would rise to 25,000 a day.
Yet despite promises, the maximum number of tests performed in one day was 8,400 on Thursday March 19. Since then the average daily figure for testing has been 4,573.
Lord Crisp, NHS Chief Executive and Permanent Secretary at the Department of Health from 2000 to 2006, told The Telegraph that the lack of testing of staff was an "obvious bottleneck" that the Government needed to address.
"We are seeing these situations where you have three junior doctors living as flatmates- one has a cough and three of them are off work for two weeks," he said. "That is ridiculous."
As of Sunday, Britain had carried out a total of 78,340 tests, around 1,164 per one million people, far behind many other countries.
In Europe's worst-hit country – Italy – health teams performed more than 3,499 tests per one million citizens (206,886 tests).
Most recent figures show that, more than a week ago, Germany had already carried out 167,009, the equivalent of 2,023 tests per one million people, while Norway has tested 6,000 per million (34,583 tests).
Meanwhile, South Korea has tested 316,664 people – equivalent to 6,148 tests per one million population, while Singapore performed 6,507 tests per one million people.
Prof Rosalind Smyth, Director and Professor of Child Health, UCL Great Ormond Street Institute of Child Health, who is currently self-isolating with suspected coronavirus, warned that Britain simply had no idea how many cases it had because of a lack of testing.
"We have really very little idea of the number of 'cases', and I am concerned that this figure is so misleading that it should not be used.
"On conservative estimates, the true figure is likely to be five to 10 times higher. It may heighten concern as people think the case fatality rate is much higher than it is, or provide false reassurance that the number of "cases" in a given area is much lower than it is."
On March 12, Britain said it would no longer be testing in the community and instead only checking people admitted to hospital for the virus, leading to widespread criticism from UK scientists.
Dr Tedros Adhanom Ghebreyesu, Director-General of the World Health Organization, criticised countries that are not testing sufficiently, warning them: "You cannot fight a fire blindfolded… Test, test, test."
He added: "To win, we need to attack the coronavirus with aggressive and targeted tactics - testing every suspected Covid-19 case, isolating and caring for every confirmed case, and tracing and quarantining every close contact."
The Royal College of Nursing (RCN) also wrote to the Prime Minister saying it was crucial to test NHS workers and provide them with adequate personal protection equipment (PPE).
In her letter, Dame Donna Kinnair, Chief Executive of the RCN, said: "Our members have been unstinting in their dedication and professionalism to protecting the health and well-being of the United Kingdom - they must be supported by the Government and health sector in return.
"Our members are coming out of retirement, students are interrupting their studies, and nursing staff are deploying from non-clinical settings, all to support the frontline in the battle against Covid-19.
"We ask you to personally intervene and act to ensure enough supply of PPE and testing for Covid-19 is available for all nursing staff and our colleagues across the health and care system."
Matt Hancock, the Health Secretary, said he hoped that the testing of medical staff would take place "as soon as possible" so that doctors, nurses and other NHS workers who may not have the virus but are self-isolating can get back to work.
He told BBC Breakfast: "We are rapidly expanding testing. We have been buying testing kits over this weekend and all of last week to make sure we have as much as possible."
He said he hoped to say more about the expansion of tests in the next few days.
Experts are also hopeful that a new antibody test, which can tell if people have been infected in the past, will be available soon to find out who is now immune.
Prof Martin Hibberd, Professor of Emerging Infectious Disease at the London School of Hygiene and Tropical Medicine, said: "When this diagnostic does become available, I think it will become a big help in tackling the disease.
"Most likely this will be reserved for frontline healthcare workers and other key workers first and perhaps vulnerable groups later, before becoming more widely available."
However, Dr Martin Marshall, chairman of the Royal College of GPs, warned there aren't enough testing kits to go around the country and officials have refused to reveal how many tests were available to the NHS.
Experts said they believe the lack of testing was less to do with unwillingness and more likely to be a lack of kits available and enough people to process the results.
Dr Bharat Pankhania, Senior Clinical Lecturer, University of Exeter Medical School, added: "Are we not testing enough? Yes, we aren't and this is most probably a capacity issue.
"We are having to pull out key workers when we're desperately need them because we're not certain if they have an infection."