Two young boys are putting the finishing touches to their twig-lined den, next to a welly-clad girl who has discovered a toad hiding by a tree. Several children are busy fishing for worms, while others are scooping up bird feathers as part of their treasure hunt.

Yet these children, playing at a "forest school" in a south-east London park, are an endangered species, according to the first ever national measurement of the extent to which children are in touch with the natural world. The three-year research project by the RSPB, published on Wednesday, shows that according to the conservation group's scoring system, four out of five children in the UK are not adequately "connected to nature".

The study, which saw 1,200 children aged 8-12 years questioned on how strongly they agreed or disagreed with 16 statements such as "I enjoy touching animals and plants", "being outdoors makes me happy" and "humans are part of the natural world", suggests that girls have a better connection with nature than boys. It also found children in London had a stronger connection than those in Wales.

Child playing in a stream in a forest in autumn. Photograph: Zak Waters /Alamy

The study follows a major stock-take of the state of Britain's nature, published in May, which found that the majority of UK species are declining, and one in three have halved in number in the past five decades. Conservationists linked the decline in wildlife to the disconnect between children and the natural world. "Nature is in trouble, and children's connection to nature is closely linked to this," said Dr Mike Clarke, the RSPB's chief executive.

He added: "This report is groundbreaking stuff. Millions of people are increasingly worried that today's children have less contact with nature than ever before, but until now there has been no robust scientific attempt to measure and track connection to nature among children in the UK, which means the problem hasn't been given the attention it deserves."

Hugh Dames, who runs the forest school in Mayow park in Lewisham, was not too surprised by the findings. "I've had children who hated to get their hands dirty, who are frightened by wood lice. But by the end of just one session, they're happily playing in the mud, looking for worms and creepy crawlies."

A girl's hands holding conkers. Photograph: Elly Godfroy/Alamy

Dames said he had been inspired by his previous work with schools and thinking about what he wanted to teach his daughter upon becoming a father 18 months ago. "If they [children] understand nature, they will value it and will take more care of it when they get older. If you're not engaged with it, you're not going to feel any need to safeguard it. I think outdoor learning is the only way to do that," he said.

The RSPB team worked with researchers at the University of Essex to devise a meaningful way of defining a connection to nature, concluding in the report that it was "enjoyment of nature; having empathy for creatures; having a sense of oneness with nature; and having a sense of responsibility for the environment."

Based on the polling, they then ranked a child's connection to nature on a scale of the lowest, -2, to the highest, +2, and considered 1.5 a "realistic and achievable" target based on children who visit RSPB nature reserves. The percentage of UK children over the 1.5 score was 21%, while girls were at 27% compared to boys at 16%. Scotland had the highest percentage over 1.5, at 27%, falling to 25% in Northern Ireland, 24% in London, 21% in England and 13% in Wales.

Suzanne Welch, education manager at the RSPB, said she could only speculate on the reasons behind the surprising finding that Welsh children were less connected than those in London. "Might it be that there are lot of accessible green spaces in London? Children in the countryside can be quite isolated with small local roads and no pavements. Access might be quite difficult in rural areas." She said that more work was needed to find out why, but she said it showed "just being in the countryside doesn't make you connected to nature."

The issue will be back in the spotlight next week, with the premiere of Project Wild Thing, a film exploring why children spend less time playing outdoors and interacting with nature. David Bond, the film's director, and a father of a six-year-old girl and four-year-old boy, said the RSPB report's findings sounded "absolutely spot on" from his work making the film, which included a visit to Eltham school, in London, where most of the children he spoke to had "other priorities" than the outdoors.

"What's important about that study to me is not the percentage but that finally it's being measured, and according to this measurement system we have a problem," he told the Guardian. "And I've got a bad feeling in a year's time it will get worse." He cited what he called well-founded fears from parents over traffic and misplaced fears over "stranger danger" as two of the culprits. Children were spending too much time on screens, such as TVs and iPads, he said, but cautioned "technology is not really a problem in itself. It's a much more fundamental commercialisation of childhood."

The RSPB said its polling created a baseline which it hopes to use to measure how children's connection to nature changes in the future. The polling was done face-to-face by TNS for 1,088 British children aged 8-12 years old, and by Ipsos Mori for 112 children of the same age in Northern Ireland.