Sports should return from the "bottom up not the top down" when the coronavirus lockdown lifts, according to a public health adviser to the World Health Organization.

Dr Brian McCloskey, who was public health director for London 2012, said community sport could be the first type of sport to return - but "bigger events will be a challenge this summer".

Professional and semi-professional leagues and competitions, as well as grassroots and amateur sports have been suspended since the middle of March.

Representatives of major sports, including football, rugby, cricket and horse racing, are meeting government medical experts this week to discuss plans to resume action.

However, in men's football, for example, levels five to eight have already ended their seasons early - while talks continue about restarting the Premier League and English Football League in June.

McCloskey considers it "much easier" to see how a local sports club or ground could reopen safely, perhaps in late spring or early summer, but cast doubt on sport higher up the pyramid.

"The bigger the match, the bigger the competition, the more complicated those mitigating actions will have to be - and therefore the less likely it is that they can be done safely," he told BBC Sport.

"So, an event that involves lots of travel across the country, or between countries, is much more complicated.

"A local event - community football, community running - is much easier to see how that happens. Bigger events will be a challenge this summer."

Other sports, including Olympic and Paralympic sports, have also been involved in separate discussions with government about financial assistance and restart plans.

Meanwhile, Sport England has launched a £20m Community Emergency Fund to assist grassroots sports groups, £8m of which has already been allocated.

'Starting locally - like Parkruns'

McCloskey thinks that sports such as marathon running or tennis should start off at local level, before increasing in numbers and scale.

The London Marathon was one of a number of spring marathons to be postponed until the autumn.

"If you look at something like road running - if we go back to Parkruns - you could start those because they don't involve a lot of travel around the country, they involve local communities and you can manage how it's done," he said.

"In doing that, you can learn how marshalling can help with social distancing in a run.

"That then helps you work out: 'How can I do a city marathon?' And, ultimately: 'How can I get the London Marathon and Boston Marathon back up again?'"

Football's restart plans?

The Premier League is hoping to resume the season on 8 June, as part of a plan dubbed "Project Restart". McCloskey said "it is possible" that will be achieved but added that contact sports, such as football and rugby, are "more difficult" in terms of trying to reduce risks.

Fifa's medical chief, Michel D'Hooghe, said he was sceptical about whether football could resume, and warned it could bring "life and death consequences".

McCloskey said that, although behind-closed-doors football would remove the risk of crowds transmitting the virus, there remained a risk regarding public perception.

"Not having big crowds going into and out of a stadium will make it easier to manage the risk, but you still have to work out how they get all of the officials in there, how you get the players in there, the trainers, the TV cameras.

"But obviously with smaller numbers it is easier to keep people separated and get them in and out easier. So that may be a way.

"There is a slight risk from the Premier League's point of view that it sort of implies that getting the money in is more important than the fans, so we have to manage that aspect of public perception."

Tokyo 2020 could still happen without a vaccine

Tokyo 2020 president Yoshiro Mori said on Tuesday that the Olympics would be "scrapped" if they could not go ahead on their new dates, starting on 23 July 2021.

Some health experts have cast doubt on whether the Games could be held next summer without a vaccine or effective drugs to treat Covid-19 being found.

McCloskey, who advises the International Olympic Committee on public health matters, said that was "vastly premature".

"We've got 15 months to go. A vaccine would be critical and there is a very good chance we will have one before we get to Tokyo in 2021," he added.

"It's a matter, again, of looking at the risks and deciding if you can make them low enough to be acceptable. Vaccine changes it completely, makes it much safer, much easier. But even without a vaccine, it is still possible to do it."