More than 1.8 million users had signed to log their symptoms or lack thereof daily, by March 31.
Almost 60% of people who tested positive for coronavirus reported loss of smell and taste, compared with 18% of those who tested negative, analysis of the data showed.
Researchers said the reports of those symptoms were much stronger in predicting a positive Covid-19 diagnosis than self-reported fever.
Public Health England research say 'there is a range of symptoms" but do not specify a loss of smell.
On the question of symptoms, NHS England's National Medical Director Steve Powis said: "The two commonest are by far cough and a fever and that is exactly why they are the two symptoms that we are if they get those symptoms."
The team behind the app developed a model to identify which combination of symptoms together could predict Covid-19 cases.
The model features a combination of loss of smell and taste, fever, persistent cough, fatigue, diarrhoea, abdominal pain and loss of appetite, and said the strongest predictor is loss of smell and taste.
Commonly accepted symptoms of Covid-19
Shortness of breath
Aches and pains
The common accepted symptoms of the respiratory disease include a fever, which NHS England says is usually a temperature of 38C or above, tiredness and a dry cough.
People are advised to self-isolate if they have a new continuous cough and/or high temperature.
The World Health Organisation (WHO) says other symptoms can include shortness of breath, aches and pains, and a sore throat.
Some people have also reported a loss of taste and/or smell, as well as abdominal pains.
Last week the WHO’s Dr Maria Van Kerkhove said there had been “quite a few” reports about people in the early stages of disease maybe losing sense of smell or taste but added “this is something that we need to look in to, to really capture to see whether this is one of the signs of Covid-19″.
Lead researcher Professor Tim Spector from King’s College, said: “When combined with other symptoms, people with loss of smell and taste appear to be three times more likely to have contracted Covid-19 according to our data, and should therefore self-isolate for seven days to reduce the spread of the disease.”
He said their research is helping to track where the virus is, before the effects are felt by the NHS, and encouraged people to continue or start logging their symptoms, even if they feel well.
Prof Spector said the data “gives us an evolving map of the UK of where symptoms are occurring two to three weeks before a strain on the NHS, which is why it’s vital to continue logging your health and symptoms, even when you feel completely healthy, and encourage others to use the app”.
Coronavirus: Everything you need to know