Children's health is suffering because they are losing the chance to play outside, a group of experts has warned.
Adults' anxieties are damaging children, the letter says
Over-anxious parents, computer games and school tests are to blame, a letter signed by almost 300 academics, authors and charity leaders says.
The decline in "unstructured, loosely supervised" play is adversely affecting children's mental health, they add.
It also threatens young people's long-term development, the letter to the Daily Telegraph says.
Among the signatories are novelist Philip Pullman, director of the Royal Institution Baroness Susan Greenfield and child care expert Dr Penelope Leach.
More than 40 professors, 60 psychologists and psychotherapists, and leaders of the main children's charities and teaching unions also lend their names to the letter.
It flags up a recent Unicef finding that British children were among the unhappiest in a league table of 21 industrialised countries.
The letter says: "We believe that a key factor in this disturbing trend is the marked decline over the last 15 years in children's play."
The experts say that play - especially when it takes place outdoors - is crucial to a child's health.
An increase in traffic, parental fears about abduction by strangers and a "test-driven" culture of education have all contributed to the trend, they insist.
They add that "the ready availability of sedentary, sometimes addictive screen-based entertainment and the aggressive marketing of over-elaborate, commercialised toys" have also played a part.
They call for a "wide-ranging and informed public dialogue about the intrinsic nature and value of play in children's healthy development".
The signatures were gathered by Sue Palmer, author of the book Toxic Childhood, and Dr Richard House, a senior lecturer in psychotherapy at Roehampton University.
They said: "'Real play' - socially interactive, first-hand, loosely supervised - has always been a vital part of children's development, and its loss could have serious implications.
"Just as the epidemic of childhood obesity recently took the developed world by surprise, too much 'junk play' could (like too much junk food) have alarming implications for the next generation."
Bob Reitemeier, chief executive of The Children's Society, said he agreed that adult anxieties about the modern world were curtailing children's freedom.
He said: "Recent research has shown that the UK is one of the worst places in the western world to be a child.
"As mentioned in today's letter, play is one of the crucial elements of a good childhood."