Telegraph Sport has launched a "Keep Kids Active in Lockdown" campaign calling on the Government to rethink its decision to wipe out children's sport from Thursday.
Epidemiologists, public health experts and cross-party MPs are all warning of a mental and physical health timebomb as activity levels plunge again among the under-18s.
Scientists insist outdoor infection risk is significantly lower than in the classrooms, which will remain open when the second Covid-19 month-long lockdown begins on Thursday.
Downing Street, however, was on Monday night continuing to resist pressure to amend its policy.
It is very difficult to take out one part of the Jenga block without disturbing the whole package," said Boris Johnson as he was asked again to consider exemptions following pressure from adult golf and tennis clubs. A Whitehall source suggested the closest a child will get to playing football will be a kickabout in the park with one child from another household.
Experts say loosening the rules on outdoor youth sports - such as football, rugby, netball and hockey clubs - is an even more urgent consideration against the rising tide of child obesity and anxiety
Telegraph Sport's campaign has won support from a cross-party group of MPs, sporting bodies and leading scientists. Damian Collins, the former chairman of the Digital, Culture, Media and Sport select committee; Tracey Crouch, the former sports minister; and Lord Triesman, the former chairman of the Football Association, published a letter calling on Oliver Dowden, the Culture Secretary, to demand a reprieve beyond Thursday.
"We believe that the risks to the spread of the coronavirus from outdoor sports would be minimal," said the letter, which also calls on the Government to include academy set-ups in the current protections for elite adult sport.
Current provisions - under which youngsters avoid changing rooms and shaking hands - had played a part in mitigating any unnecessary risk since amateur sport was phased back over the summer, scientists added. Carl Heneghan, a professor and epidemiologist at the University of Oxford's Centre for Evidence-Based Medicine, said the blanket threat facing youth sports "is simply unacceptable".
"Children's sports should be a priority, unless we want a generation more of an obesity problem, more of a diabetes problem in the future," he said. "It's very clear the habits you create as a child are taken into adulthood. Participating in outdoor sports has to be preserved for children at all costs. They are incredibly low risk, and no risk to children themselves."
Alison Oliver, chief executive of the Youth Sport Trust, is among a host of sporting leaders backing The Telegraph's campaign. "Maintaining access to sport and weekly physical education is vital to young people's health and well-being," she added. "During the first national lockdown almost half of young people were less active. It led to children becoming physically unfit and many struggling with their mental health."
The Rugby Football Union also expressed support, adding it was "really important for children and young people to stay active". "Provided it is safe to do so, we would encourage the Government to continue to allow children to participate in rugby activity at their clubs," a spokesman said.
An obesity crisis facing school-aged children was looming even before the first lockdown, according to the Government's own figures. More than a quarter (27.5 per cent) of year six children, aged 10-11, living in the most deprived areas were obese compared to 11.9 per cent of those living in the least deprived areas, the Government's National Child Measurement Programme for the 2019-20 school year found.
"It is so important to get the policy on sport changed," Prof Heneghan added. "How can we stop this while the Premier League is carrying on?"
Ambiguity and confusion for teachers over the risk of Covid infection inside and outdoors at schools have already led to many schools scaling back contact sports or abandoning them altogether during PE classes.
Prof Gabriel Scally, president of the epidemiology and public health section of the Royal Society of Medicine, recognised the situation facing ministers was complex, but said the risk of transmission during outdoor contact sport would not necessarily be higher than in the classroom.
"When people are playing most sports, they are exerting themselves and therefore breathing heavily," he said. "And if it involves verbal interactions, shouting and so on, which could all generate more aerosol emissions. It could act to spread the virus. But compared with sitting in a sealed classroom, and in significant numbers for a long time, it certainly would be absolutely no worse than that."
The FA, which moved into action to ensure the 10 non-elite teams left in the FA Cup could play in the first round later this week, is understood to be in dialogue with the Government over amateur youth matches. Whitehall sources played down any hopes, however, saying it was unlikely any further exemptions would be made.
England Hockey, meanwhile, pointed out other countries had spared youth sport from the toughest measures. "In other countries, there has been dispensation for under-18s to play hockey during national lockdowns; for example in the Netherlands, and this has proven to be effective," a spokeswoman said.
Several studies suggest the loss of youth participation in sport has been far more damaging to society than the loss of crowds at elite competition.
Dr Chris Mackintosh, who has been studying the impact of lockdown restrictions in the North West since March, said the closure of sporting clubs had been particularly damaging for youngsters from lower-income families or those with a chaotic home life. His study for Manchester Met University also found a disproportionately high drop-off in participation among the grass roots, rather than in the elite.
"The social function of sports is one of the main things that people go to for sports," he said. "The removal of sports as an area of social life is what people have found hardest."
Two petitions - including one which appeared to have been started by the former footballer Robbie Savage - had gathered tens of thousands of signatures on Monday calling for youth sports to be spared.