The problem has become so acute that hundreds of thousands of children are being prescribed a controversial drug, branded a 'chemical cosh' by critics, that is so effective at calming troubled children that it reduces them to a zombie-like state.

New figures obtained by The Observer show that the prevalence of Attention Deficit Disorder (ADD) and the related Attention Deficit Hyperactivity Disorder (ADHD) show that 345,000 children, or one in 20 aged between six and 16, have the conditions.

The estimate is much higher than previously thought and is based on new studies predicting that in a class of 30 children between one and three will have the disorders, which causes uncontrollable and sometimes violent behaviour.

The figures also show that the number of Ritalin prescriptions has soared by 126 per cent over four years. In 1997 doctors were writing 921,000 prescriptions a year, but last year this rose to 2,085,000, at a cost of £3.85 million to the NHS.

Ritalin, whose non-proprietary name is methyl-phenidate, is given to help children to concentrate on schoolwork and calm down. Since its first appearance it has been surrounded by controversy.

Opponents have accused the pharmaceutical industry of over-sedating children and say that the amphetamine-like drug can act on the brain in the same way cocaine does. However, many parents and psychiatrists say the drug has been a lifeline, transforming the behaviour of children, allowing them to get on at school and preventing the break-up of families.

There is also concern that some children may be prescribed the drug without proper assessments, or without their treatments being reviewed properly after being on the medication for a month or more.

Liberal Democrat MP Paul Burstow, who obtained the figures via written questions to Health Minister Jacqui Smith, said: 'I am concerned that the prevalence of these disorders seem to be on the rise. There is a very big debate about the rights and wrongs of Ritalin, but we need to look at why the prescription rates have gone up so steeply. It has become the option of first choice for some families, but it needs to be given appropriately, and only if it is really necessary.'

He added that there were still questions over the impli cations of giving such a powerful drug to very young children. 'It is only now that Ministers are beginning to quantify the problem. We need a coherent strategy so that all the professionals involved with children - from teachers to social workers to GPs and nurses - are alert to the signs and know what advice and support to give to children and parents,' he said.