Attention deficit hyperactivity disorder (ADHD) is being missed in girls because they may not display the same challenging behaviour as boys, experts say.

New guidance from the National Institute for Health and Care Excellence (Nice) warns that some groups with ADHD are being overlooked and calls for more scrutiny to ensure people get the right help.

Up to 5% of school-age children are thought to suffer from ADHD - a condition characterised by a group of behavioural symptoms including hyperactivity, difficulty concentrating and impulsiveness.

Dr Gillian Baird, professor of children's neurodisability at Guy's and St Thomas' NHS Foundation Trust and chairwoman of the Nice guideline committee, said girls are thought to be under-diagnosed with the condition, while some are given an incorrect diagnosis.

She said: "Among the possibilities are that boys present with more obviously disruptive behaviour."

The new guidance also calls on parents of children with ADHD not to put them on special diets, such as eliminating nuts, milk and wheat, or cut out artificial colours in a bid to improve behaviour.

Some parents restrict diets for their ADHD children - known as "few food" diets.

Research funded by the Food Standards Agency (FSA) in 2002 also found that consuming some artificial food colourings and the preservative sodium benzoate could be linked to increased hyperactivity in some children.

But in the new guidance, Nice says doctors should not "advise elimination of artificial colouring and additives from the diet as a generally applicable treatment for children and young people with ADHD".

It said that parents who think there is a link between poor behaviour and diet should be advised to keep a diary, while a dietitian and mental health specialist should be involved before any restrictive diets are introduced.

Doctors should also not advise parents to routinely give their children fatty acid supplements, and parents should be told there is "no evidence about the long-term effectiveness or potential harms of a 'few food' diet for children with ADHD, and only limited evidence of short-term benefits".

The guidance also says doctors should be informed that some groups are more likely to have ADHD than the general population.

This includes those born premature, children in care, children with issues such as depression, and those with a close family member diagnosed with ADHD.

People with epilepsy are also more at risk, as are those who have other conditions, such as autism.

The guidance also now recommends drug treatments including Ritalin for children and young people with ADHD where the condition is having a persistent impact on their daily life. In previous guidance, Nice said these should not be routinely prescribed to youngsters.

However, the new guidance says the drugs should only be prescribed by a specialist in ADHD rather than by family doctors, with regular reviews.

Professor Mark Baker, director of the centre for guidelines at Nice, said: "Our draft guideline raises awareness of people who more likely to be wrongly diagnosed with ADHD, those with ADHD who are wrongly diagnosed with another condition and those whose ADHD is missed altogether.

"Not having an accurate diagnosis can have a negative effect on people and their families. It means they cannot access the best treatment and support."