One in five children are overweight or obese by the time they start primary school – and one in three reach this stage by the end of it, NHS data shows.The coalition of fitness and sports experts said that for just £7 a day, per child, ministers could boost activity levels among 1.2 million school-aged children.Baroness Grey-Thompson said: “It is time for the Government to show its commitment to the next generation by unlocking the school sports facilities lying unused on the doorstep of every neighbourhood.“We urge the next Prime Minister and Chancellor to support schools to have a powerful impact on the health and wellbeing of children, young people and their families by opening over the summer holidays and engaging families in local, affordable and healthy activity opportunities.“This is an unmissable opportunity to reshape the summer holiday period, and realise the ambitions of many existing policies aiming to improve child health by encouraging physical activity, tackle childhood obesity and reduce loneliness through improved social cohesion.” Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings. Schools should be kept open all summer, running sports clubs, in a bid to tackle Britain’s growing obesity crisis, fitness leaders say.They are urging ministers to fund a new programme which would give children access to sporting facilities and playing fields throughout the holidays.Sports leaders are calling on the Treasury to take action to tackle record levels of obesity and inactivity.It follows research showing that today’s children are far less active during summer holidays than term-time, with fitness gains achieved during the school year wiped out in just six weeks.Research on more than 400 children found that during the academic year, pupils made steady improvements in their fitness levels. But around 80 per cent of gains were reversed during lazy summer holidays, in which PE lessons and walks to school were replaced with long days hunched over gadgets.Today 50 fitness experts including former England rugby captain Lawrence Dallaglio, and Baroness Grey-Thompson, one of Britain’s most successful paralympians, urged ministers to invest in summer sport.Almost 40 per cent of all sports facillities in England sit behind school gates – but most of these cannot be accessed during school holidays, they warned.The coalition of sports leaders is urging the next Tory administration to allocate funding to such a programme in the next Spending Review, saying it would take “minimal investment” to make better use of school facilities. Couch potato childhoods will cost lives, and burden the NHS in future years, warn the group, led by Baroness Tanni Grey-Thompson, ukactive’s chairman. The plea is backed by politicians including former Conservative sports minister Tracey Crouch, Labour deputy leader Tom Watson, and Damian Collins, chairman of the Commons Digital, Culture Media and Sports committee.It is also supported by leading health experts including Shirley Cramer, chief executive of the Royal Society for Public Health as well as sports figures, including former featherweight world boxing champion Barry McGuigan.In the submission – which will be sent to the Treasury after a leader is elected – the coaltion urge ministers to grasp “an unmissable opportunity” to tackle obesity and boost childrens’ fitness.“Today’s children are the least active generation ever,” they warn, highlighting research showing that one third of children do not even achieve half an hour’s physical activity daily.It comes as a poll found that 85 per cent of people support the proposal to open school facilities during the holidays. The survey of 2,000 adults by Com Res also found 87 per cent of parents and guardians say they would consider using them for clubs offering children physical activity through sports, dance and other activities.