Findings from the government-backed REACT-2 study found that more than one in three people who had COVID-19 experienced one or more symptoms lasting at least 12 weeks.
This group accounted for 5.8% of the study population - suggesting that across England's adult population more than 2m people may have experienced long COVID.
The findings come just days after NHS England published details of a long COVID enhanced service for general practice, worth £30m nationally.
The package is intended to fund staff training on how to identify, assess and manage long COVID - as well as supporting patients to self manage, getting to grips with local clinical pathways for the condition, coding data and developing an 'equity of access plan'.
However, the latest research findings suggest that at an average GP practice in England with 9,000 patients in total, 400 adults or more could have experienced long COVID.
The REACT-2 study is based on data from 508,707 adults aged 18 and above and was carried out over several rounds between September 2020 and February 2021.
Researchers said about one in five of those surveyed reported having had a COVID-19 symptom previously, with over a third reporting persistent symptoms lasting at least 12 weeks. Around one in 10 those with symptoms said they lasted at least 12 weeks and were severe.
The study found that prevalence of long COVID rose with age, becoming 3.5% more likely for every 10-year increase in age. It found that the condition was more common in women, people who are overweight or obese, people who smoke, live in deprived areas, or had been admitted to hospital - but that long COVID was less prevalent in people of Asian ethnicity.
The results came as another study from University College London and Kings College London researchers showed that 17% of middle-aged people who reported being infected by SARS-CoV-2 also reported long COVID symptoms. Among younger adults this fell to 7.8%, the study found.
Professor Paul Elliott, director of the REACT programme at Imperial, said: 'Our findings do paint a concerning picture of the longer-term health consequences of COVID-19, which need to be accounted for in policy and planning.
'Long COVID is still poorly understood but we hope through our research that we can contribute to better identification and management of this condition, which our data and others’ suggest may ultimately affect millions of people in the UK alone.”
The REACT-2 study found that people with long COVID fell into two main groups - with one experiencing symptoms including tiredness and muscle aches, while a second group reported 'shortness of breath affecting normal activities, tightness in chest, and chest pain, with more people reporting that they had severe symptoms'.
Responding to the latest data, health and social care secretary Matt Hancock said: 'Long COVID can have a lasting and debilitating impact on the lives of those affected. Studies like this help us to rapidly build our understanding of the impact of the condition and we are using these findings and other new research to develop support and treatments.'
More than 80 long COVID assessment centres have been opened across England, the government says, and plans have been set out for 15 specialist centres for children with the condition