The true number of global coronavirus infections may be up to six times greater than the reported number of cases, analysis has found.
The modelling study used data from 15 countries to estimate the true number of infections across a combined population of more than 800 million people.
Italy, the UK, France and South Korea were among the countries studied.
Researchers found that in Italy, estimated cases were 17.5 times higher than recorded, but just 2.6 times higher in South Korea.
In some countries - including the UK, Italy, France and Belgium - the reported number of cases represented less than 10 per cent of the true number, the found.
Australia had the best level of detection among the 15 countries at the end of April, the team said. But they added the rate of infection there may have been five times higher than what was officially reported at the end of August.
Professor Quentin Grafton, of Australian National University, said: "We found Covid-19 infections are much higher than confirmed cases across many countries, and this has important implications for both control and the probability of infection.
"Our analysis has found more than 5.4 million in the UK - 8% of the population - are or have been infected with the coronavirus."
He added: "These findings raise serious questions about how we deal with all facets of the coronavirus pandemic, including ongoing morbidity and life-long health impacts for people who have been infected, how we implement and manage lockdowns, and how we make sure we are on top of this pandemic more broadly."
The researchers used a method known as "backcasting", where they examined Covid-19-related deaths and compared this with the time from infection to symptoms and time from symptoms to death.
The authors said this method allows them to provide a 95 per cent confidence interval around their estimated population infection rate.
Analysis showed that infection rates between March and August across the 15 countries were, on average, 6.2 times greater (49.7 million) than reported cases (eight million).
Dr Steven Phipps, from Ikigai Research in Australia, said: "Simply put, we analysed statistics on how many people had died from Covid-19 in a given country and then worked backwards to see how many people would have to have been infected to arrive at that number of deaths.
"Our method is a novel and easy-to-use method for estimating the true infection rate wherever there is reliable data on the number of fatalities attributable to Covid-19."
The research is published in the journal the Royal Society Open Science.