Old age and having a wide range of initial symptoms increase the risk of "long Covid", say scientists.
The study estimates one in 20 people are sick for least eight weeks. The research at King's College London also showed being female, excess weight and asthma raised the risk.
The aim is to develop an early warning signal that can identify patients who need extra care or who might benefit from early treatment.
The findings come from an analysis of people entering their symptoms and test results into the COVID Symptom Study app.
Scientists scoured the data for patterns that could predict who would get long-lasting illness.
"Having more than five different symptoms in the first week was one of the key risk factors," Dr Claire Steves, from Kings College London, told BBC News.
COVID-19 is more than just a cough - and the virus that causes it can affect organs throughout the body. Somebody who had a cough, fatigue, headache and diarrhoea, and lost their sense of smell, which are all potential symptoms,- would be at higher risk than somebody who had a cough alone. The risk also rises with age, particularly over 50, as did being female.
Dr Steves said: "We've seen from the early data coming out that men were at much more risk of very severe disease and sadly of dying from Covid, it appears that women are more at risk of long Covid."
No previous medical conditions were linked to long Covid except asthma and lung disease.
Source: BBC News, 21 October 2020