Professor Ball, who researches playground safety for the Health and Safety Executive, says playgrounds are and should be risky.

"Unfortunately playgrounds are being dumbed down, largely out of fear of litigation but we need to keep an eye on what playgrounds are all about.

"Some researchers would have all playground equipment below 1.5m but this conflicts with the notion of play value - of giving children excitement, enjoyment and a risk which they can learn to deal with," he said.

The idea of colour-coding playgrounds does not win the approval of the parents' organisation the National Confederation of PTAs.

The organisation's Margaret Morrisey says school playgrounds are already subject to close inspection and there are enough regulations.

"In general, playgrounds are very safe. If you went down this road, the costs would be prohibitive and perhaps schools would say they couldn't cope and then we would not have playgrounds.

"The most important thing is the safety of children, but we need children to have a challenging environment, although not a dangerous one, in order to learn."

Parental judgement

For playground safety expert Peter Heseltine at the Royal Society for the Prevention of Accidents, grading of playgrounds would be a notice too far.

"There are already enough notices in playgrounds, " he said.

"And the problem with this is it takes the responsibility away from parents, who are perfectly able to make judgements in respect of their own child.

"Some children are more confident and better coordinated than others."

Mr Heseltine said another problem was that such zoning would give a false high expectation of safety and could lead to more legal claims.

Professor Ball said the idea of taking away parental responsibility was "nonsense".

He said playgrounds were risky places and that parents should not think they were places they could just deposit their children and pay no attention to them.