A report from Public Health England says its review of UK and international evidence suggests obese and excessively overweight people are at a higher risk of coronavirus death or serious outcomes.
However, weight does not appear to affect a person's chances of contracting COVID-19.
One study cited found a 40% increased risk of death for those with a BMI of 30-40.
Another study found that 7.9% of COVID-19 ICU patients had a BMI over 40 compared with 2.9% of the general population
However, the report does caution that there are some limitations with current evidence on obesity and COVID-19, including around the effect of weight loss on reducing risks.
New Obesity Measures Anticipated
Sixty-three per cent of adults in England are above a healthy weight, with 36% overweight and 28% obese. The PHE report has been published ahead of anticipated Government anti-obesity measures being announced for England next week.
Just last month the PM acknowledged that the British are far fatter than any other nation in Europe except Malta and it has been reported that Boris Johnson's previous opposition to 'nanny state' weight loss measures may have been changed by his personal ICU coronavirus experience.
Last week, health experts and charities appealed to the PM to implement all outstanding recommendations in the Government's childhood obesity prevention plan.
Meanwhile, Sky News quoted a letter from the Institute Of Practitioners In Advertising saying an expected ban on TV junk food adverts until after the 9pm watershed would be "draconian".
PHE says excess fat can affect the respiratory system, and is likely to affect the body's inflammatory and immune functions.
The report also says some of the additional risk in BAME communities and some older adults may be linked to obesity.
Those who are obese may be also less likely to seek health care and support, the report says.
PHE says exercise levels did not increase overall under lockdown, and pointed to increased sales of alcohol and snack products.
Commenting in a news briefing hosted by the Science Media Centre, Susan Jebb, professor of diet and population health, University of Oxford, said: "It's very clear that being overweight is associated with an increased risk of hospitalisation and also of death, and that those risks are pretty linear. They go up really from a BMI of about 20, so anything above the healthy range increases risk."
She added: "Although this evidence is still quite early days, it's all incredibly consistent, all points in the same direction."
Even alongside other health conditions, weight was the most significant risk she said. "We've heard that people with diabetes, for example, are at increased risk and that's absolutely true, but in many cases, part of that increased risk is through being overweight. So when you adjust for overweight, actually the risk of having diabetes is much, much smaller. So obesity is the big risk factor."
Naveed Sattar, professor of metabolic medicine, University of Glasgow, said COVID-19 could be the prompt some people needed: "This pandemic has scared people. If this helps some people improve their lifestyle to make some small sustainable changes in a positive direction, I think that's a good thing, but we mustn't give people a hard time about this."
"I think we need a two-pronged approach. One is to help people avoid gaining weight, and that means big population-level changes, so that we have less advertising of foods, less promotion of unhealthy foods," she said.
"But what we also need is encouragement and support for people who are overweight to lose weight, and support really matters," she added.
"This pandemic has shone a light on obesity," Prof Sattar said, "in a way that perhaps people didn't recognise is really important."
Prof Jebb added: "It's a great 'reset moment' for everybody just to think about their lifestyle, to think about a small number of things that maybe they could do differently, which will help them to manage their weight. It's a unique moment in time, I think."