Nearly half of British children are overweight because of a lack of fitness exacerbated by lockdowns induced by the pandemic.

A study done in partnership with researchers from the University of South Australia and Western Australia's Murdoch University found children's body mass increased by an average of 6.8 kilograms over a one-year period, about twice the amount expected.

Data, including body mass index and results from a series of physical exercises, from 178 children in Newcastle-upon-Tyne aged between eight and 10 was collected from October 2019 until November 2020.

Researchers found 51 per cent of children were unfit and 47 per cent of children were overweight or obese.

UniSA researcher Naomi Burn said the restrictions imposed, such as a maximum of one-hour a day of outside activity, adversely impacted children's physical health and well-being.

"When COVID-19 hit the UK in 2020, infection control measures led to the closure of schools for most pupils, outdoor playgrounds and sports clubs closed," she said.

"Such unprecedented restrictions have had a distinct impact on children's physical and mental health, with nearly half of children presenting as being obese and more than half classified as unfit.

"While the pandemic persists, we need to recognise the need to keep kids healthy and active. Not only will this benefit them now, but also later in life."

Governments and schools needed to invest in sports programs to help children regain fitness, Dr Burn said.