Loss of the senses of taste and smell has been defined as one of the key symptoms of coronavirus under new Government guidance.
The development comes as research shows it is the best indicator for the virus, with two in three sufferers experiencing changes in those senses.
Scientists criticised health officials for being slow to issue the new advice, saying far too many people have been circulating, and been encouraged to go back to work, when they should have been self-isolating.
Until now, the rules to classify cases have said the virus should only be suspected if a person has a new continuous cough or fever, but the new clinical advice adds “anosmia” – the loss of taste or smell – to the list.
Anyone suffering a loss of taste or smell, or a noticeable change to those senses, should self-isolate for seven days to reduce the risk of spreading the infection, according to the guidance from the UK’s chief medical officers.
Those sharing a household with someone with such symptoms should stay at home for 14 days.
It comes as a British study found that 65 per cent of those who tested positive for coronavirus had reported loss of smell and taste – three times as many as those who tested negative.
The research, by King’s College London, involved 2.6 million Britons, including more than 18,000 who underwent tests for the virus. Changes in smell and taste were far more reliable indicators of Covid-19 than symptoms such as persistent cough or fever, it found.
While 57 per cent of those who tested positive for the virus had a persistent cough, so did 47 per cent of those found to be negative. In total, 34 per cent of those with fever tested positive, along with 24 per cent of those who were negative.
Professor Tim Spector, the lead researcher, criticised the Government’s slowness to respond to growing evidence about symptoms such as the loss of the sense of smell.
He said infected people had been encouraged to go back to work, because of the failure to properly examine the symptoms which best indicate that a person has coronavirus.
Prof Spector asked health officials to reconsider their advice again, and take account of a host of other symptoms, such as fatigue and muscle pain, that could indicate the presence of the virus.
He said that between 50,000 to 70,000 Covid-19 sufferers in the UK were being excluded from the advice to self-isolate, until the guidance changed on Monday.
“We list about 14 symptoms which we know are related to having a positive swab test,” he said. “These are not being picked up by the NHS. This country is missing them all, and not only underestimating cases but also putting people at risk and continuing the epidemic.
“Our data shows that at least 42 per cent of people who go on to test positive never developed a fever or a cough. However, this is likely an underestimate as the Government was only inviting people with a fever or a cough to have a swab test. We believe it’s closer to 50 per cent.”
Professor Jonathon Van-Tam, the deputy chief medical officer for England, on Monday defended the decision not to act more quickly.
Defending the time taken to reach the decision, he said that while there had been “a signal” about the possible importance of the symptom for some time, establishing its use in detecting more cases had been “quite a difficult piece of science”.
He added that estimates about how common the symptom were had been “very variable”, with the health advice only changing now because the data considered by Government advisory group Nervtag (the New and Emerging Respiratory Virus Threats Advisory Group) had “grown and become stronger”.
The change to the definition should mean around 94 per cent of cases could be identified, up from 91 per cent using previous classifications, Prof Van-Tam added.
He said it was not clear whether loss of sense of taste and smell was normally an early symptom of coronavirus, adding that some research suggested women were more likely than men to suffer anosmia.
Asked why anosmia had not been added to the list of symptoms earlier, Prof Van-Tam said Public Health England research on 229 lab-confirmed cases found only 0.44 per cent reported it without any other symptoms.
Speaking just before the new advice was issued, Prof Spector said: “Someone has got to urgently ask this question of why we’re the only country in this crisis that isn’t really widening our group of symptoms and get on with it and do something.”
He said 17 other countries had now changed their advice, as had the World Health Organisation, the European Union and the US Centers for Disease Control and Prevention in order to take on a broader range of symptoms.
“At the moment, people are being told to go back to work if they are a care worker and they have got something like loss of smell and taste, severe muscle pains and fatigue things that we know have shown are related to being swabbed positive.
“This country is missing them all and a underestimating cases but also putting people at risk and continuing the epidemic,” he added.
He also urged all Britons to sign up to use the new tracing app in order to track symptoms across the UK.
Prof Nirmal Kumar from ENT UK, the body representing ear, nose and throat doctors, said the change was “better late than never”.
He said: “We had been asking for this almost eight weeks ago. The delay has not helped at all.”
It came as a study of more than 4,000 patients with a diagnosis of Covid-19, by the University of East Anglia, found they suffered an 89 per cent and 76 per cent drop in smell and taste respectively.
Salty taste was the most common type of taste to be reduced or changed.