Young people should be able to ring a hotline to report adults who are threatening their right to play outside, according to a report today warning that young people are being increasingly excluded from public spaces.

A study by the thinktank Demos finds that public areas in Britain are frequently "built around the convenience of the car and the shopping trip", and are "actively antisocial to children".

There should be scope for children and young people to have a far greater say in planning to ensure their needs are met, says the report, Seen and Heard, which urges adults to take a more positive attitude to children's use of public space.

A more playful approach to urban life would benefit all ages, it adds, proposing the creation of "iconic play spaces at high profile locations" in the wake of the success of the temporary grassing over of London's Trafalgar Square and of a street in Leeds, and the creation last summer of a public beach in a disused car park in Bristol.

Highlighting the loss of the street as a community area partly as a result of increasing - and faster - traffic, the study also recommends a 20mph speed limit where streets are shared with children.

The report comes amid growing concern over a decline in children's outdoor play, driven by factors including parental fears of risk from traffic or strangers, greater car use, and inadequate or unchallenging play facilities.

It argues that a lack of play opportunities, whether in structured playgrounds or in other public spaces including the street, can affect children's mental health and happiness as well as contributing towards the UK's growing obesity crisis.

A co-author of the report, Celia Hannon, of Demos, said: "With cars outnumbering children by three to one, the acceleration of house building and the privatisation of public space, places once used by young people for playing and exploring rites of childhood are being quickly swallowed up.

"Unless young people are in structured activities or acting as mini-consumers, we assume that they are causing trouble. Our streets, squares and parks need to be accessible and enjoyable for all, otherwise existing anxiety around anti-social behaviour will get worse. It's time to open up our towns and cities for all and make them more playful."

The study, commissioned by Play England, the quango charged with promoting play, is based on nine months of interviews with young people, professionals and policy makers in six areas of the UK. It examined spaces including a vandalised park with run-down play areas and nothing for young people to do, an estate where the car takes precedence over children and a town centre where common space is not shared with young people.

Across Britain, researchers found, children - outside the places designed especially for them - are at the bottom of a "user hierarchy" of public space that "seems to be unconsciously assumed across Britain. Over the past decades, two other uses of the public realm have been consistently privileged above play: cars and commerce."

Adult attitudes have also changed, becoming more intolerant of behaviour such as hanging around, which young people regard as normal, the report found. "The line between what constitutes behaviour that is unacceptable and the social behaviour of young people has become difficult to distinguish, and the intolerance of adults is now one of the most powerful factors that limits the freedom of children and young people in the public realm."

Children have a right to be in public space in an "informal and unorganised way", as well as to have set play areas, says the study, which calls for the appointment of youth planners to "youth proof" development proposals and audit public places to identify areas in need of investment.

An antisocial behaviour hotline would allow young people to point the finger back at adults where they felt their right to use public spaces was being infringed. Demos suggests the line might be staffed by local authorities.

It also proposes "work swaps" so that employees from the many agencies whose work impinges on children and public space could gain work experience in other sectors, for example moving between transport and children's services departments or green space management and the police.

Adrian Voce, director of Play England, said: "This report addresses one of the most serious issues we face as a society: the disappearance of children and young people from public space. The consequence is a decline in their opportunities for play, recreation and their own social and cultural lives other than through electronic media or highly structured activities."