Researchers from King's College London, using data from the Covid Symptom Study app, say that hundreds of thousands of people in the UK could be experiencing symptoms lasting more than four weeks and sometimes for months after the initial infection.
The study found four factors that might lead to long-term symptoms:
Women - females under 50 were 50% more likely to get Long Covid than men in the same age group
Weight - people developing long Covid had a slightly higher average body mass index (BMI) than those experiencing a short illness.
Those with more symptoms in the first week of their illness were more likely to suffer long-term.
People with asthma were more likely to develop long Covid, although there were no clear links to any other underlying health conditions.
And those experiencing more than five symptoms during the first week of illness were more than three times more likely to experience Long Covid.
One in 20 people with Covid-19 are still suffering symptoms eight weeks later and one in 50 are still struggling after three months, the study found.
Symptoms of Long Covid include ongoing fatigue, ongoing loss of taste or smell, respiratory and cardiovascular problems and mental health issues.
GP Dr Sarah Jarvis told ITV News: "The single commonest symptom is profound tiredness, absolute bone-weary exhaustion which isn't made better by rest and that is one of the key features.
"But shortness of breath, cough, headache, problems sleeping, mood changes, depression, anxiety and so on as well as changes to your sense of smell and changes in your gut, they can all happen. And palpitations and so on."
The study, led by Dr Claire Steves and Professor Tim Spector, included data from 4,182 Covid Symptom Study app users who tested positive for Covid-19 following a swab and who have been logging their health on an ongoing basis.
On Wednesday, Health Secretary Matt Hancock issued a new film on how people are living with the long-term effects of coronavirus.
It features the stories of four people left unable to live normally after Covid-19 and calls on the public to continue to wash their hands, cover their face and make space to control the spread of Covid-19.
Mr Hancock said: “I am acutely aware of the lasting and debilitating impact long Covid can have on people of all ages, irrespective of the seriousness of the initial symptoms.
“The findings from researchers at King’s College London are stark and this should be a sharp reminder to the public – including to young people – that Covid-19 is indiscriminate and can have long-term and potentially devastating effects.
“The more people take risks by meeting up in large groups or not social distancing, the more the wider population will suffer, and the more cases of long Covid we will see.”
Those with long Covid were twice as likely to say their symptoms had come back again after initial recovery compared with those having a short illness (16% compared to 8.4%).
It comes as other data from Public Health England (PHE) suggests around 10% of people with Covid-19 not admitted to hospital reported symptoms lasting more than four weeks.
NHS England chief executive Simon Stevens told a conference of health professionals on Wednesday: “It’s a real thing. It looks as if perhaps 10% of people who’ve had coronavirus have lingering symptoms for up to three months after having had Covid.
“And then a smaller minority, some estimates say about 60,000 people across the country at the moment, have got symptoms that persist three months and beyond.”
Prof Spector said the new King’s study showed that as well “as worrying about excess deaths, we also need to consider those who will be affected by long Covid if we don’t get the pandemic under control soon.”
He added: “As we wait for a vaccine, it is vital that we all work together to stem the spread of coronavirus via lifestyle changes and more rigorous self-isolating with symptoms or positive tests.”
Dr Jarvis said: "We have got quite a lot of patients who have got ongoing symptoms at six months.
"It partly depends on how severely unwell they were.
"For people who have been into intensive care, it could take well over a year."
She added: "In terms of doing things for ourselves, I'm afraid it is back to the basics - Hands, Face, Space. The less likely you are to catch it, the less likely you are to get Long Covid."
Professor Stephen Powis, NHS medical director, said: “As we continue to learn more about Covid-19, it is clear that a significant minority of patients are suffering the aftereffects for weeks or months after contracting the virus.
“New specialist centres across the country will see respiratory consultants, physiotherapists, other specialists and GPs, all help assess, diagnose and treat patients who are suffering, and so it has never been more important that everyone does what they can to reduce the risk of spreading the virus by following the Hands, Face, Space guidance.”
Health minister Lord Bethell said: “The evidence is worrying – Covid-19 is clearly having a long-term impact on some people’s physical and mental health.
“We are moving quickly to stand up rehabilitation facilities and recovery services.
“These are becoming more accessible with the opening of specialist clinics across England.
“The NHS England long Covid taskforce will have a big impact, bridging between our research and the care people need.
“But the public must continue to be aware their behaviour has a huge impact on the spread of this virus and they must take the necessary precautions.”