Over-anxious parents, video games and "test-driven" education are conspiring to rob children of the vital opportunity to play outdoors, leading experts warn today.

A group of almost 300 teachers, psychologists, children's authors and other experts fear the erosion of "unstructured, loosely supervised" play is dangerously affecting young people's health.

They have used a letter to a national newspaper to highlight their concerns, a year after a similar missive spelt out fears that childhood was being "poisoned" by the modern world.

That unprecedented letter sparked a fierce debate on the issue and now experts hope to bring the subject of play to the public's attention.

Among those who have signed the latest note are novelist Philip Pullman, Baroness Susan Greenfield, director of the Royal Institution, and child care expert Dr Penelope Leach.

There are also 60 psychologists and psychotherapists, more than 40 university professors, and leaders of the main British teaching unions and children's charities.

In the latest letter, the experts say research in the last year has bolstered the case that Britain's children are facing a mental health crisis.

They point to examples such as a Unicef finding that Britain's children are among the unhappiest in the developed world.

"We believe that a key factor in this disturbing trend is the marked decline over the last 15 years in children's play," the letter says.

The experts say that play, particularly outdoors, is vital to a children's all-round health and well-being. "Many features of modern life seem to have eroded children's play," the letter continues, including increased traffic, parental anxiety about "stranger danger" and a test-driven school curriculum.

They also point to "the ready availability of sedentary, sometimes addictive screen-based entertainment and the aggressive marketing of over-elaborate, commercialised toys".

The letter goes on to call for a "wide-ranging and informed public dialogue about the intrinsic nature and value of play in children's healthy development."

Writer Sue Palmer, author of a book called Toxic Childhood, and Dr Richard House, a senior lecturer in psychotherapy at Roehampton University, organised the letter.

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.

"We hope this letter helps draw the attention of parents, politicians and the general public to the importance of giving the next generation time, space and encouragement to go out to play."

Bob Reitemeier, chief executive of The Children's Society said the letter was a timely reminder of the "current crisis of confidence in childhood".

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.

"Our research has revealed that children's freedom to play out with their friends is being curtailed by adult anxiety about the modern world."

The letter was sent to The Daily Telegraph, which also published last year's missive.