· Digital method could be used on two-year-olds · Asperger's syndrome may be target for technique

Tens of thousands of children with rare genetic disorders could be diagnosed earlier and more cheaply following pioneering research that uses computers to analyse images of people's faces.

The technique will help doctors make a swift diagnosis by identifying the most likely genetic disorder a child has purely by examining their features.

Until now, doctors have had to run a series of expensive and time-consuming genetic tests before being sure of their diagnosis.

The British research, published today, is also expected to allow doctors to screen children as young as two years old for autism disorders, such as Asperger's, boosting their chances of receiving appropriate care and treatment as early as possible.

The breakthrough exploits the fact that of 5,000 documented genetic disorders, around 700 cause slight but specific changes in facial development. One of the most dramatic examples is Down's syndrome, but many other conditions lead to more subtle differences in facial appearance that can only be spotted by experienced specialists.

One condition, called fragile X syndrome, is the most common inherited cause of mental impairment and is triggered by a mutation in a single gene on the X chromosome. Of the one in 4,000 newborns it affects, many develop mental impairment, autistic behaviour, anxiety problems and seizures, but they also have a subtly different appearance, growing slightly longer, narrow faces and more prominent ears.

Peter Hammond, at the Institute of Child Health in London, used 3D digital photography to build a library of healthy children's faces and merged them to produce an "average" healthy face. He then travelled to hospitals around the world to take 3D images of children with various genetic disorders and from them created typical faces for each condition. Each image contains 25,000 points that capture the most subtle contours of the face.

Dr Hammond's team has now used the images to diagnose disorders in children. Doctors simply need to take 3D images of the child's face and use the computer to see which condition their facial features most closely resemble.

The researchers are working on 30 different genetic conditions which alter the shape of the face. Among them is Williams syndrome, a mental impairment that affects one in 15,000, and is characterised by a short, upturned nose and small jaw. Children with the condition often find their behaviour is altered too, making them unusually friendly towards strangers.

Another condition, Smith Magenis syndrome, gives one in 25,000 children flat-looking noses and raised top lips. They are prone to self-harming and release the sleep hormone melatonin during the day, making them sleep in the daytime and rise at night. Many of these children develop curved spines and heart and kidney problems which can be picked up more quickly if the child is diagnosed at a young age.

In tests, Dr Hammond's computer system accurately diagnosed 92% of fragile X syndrome cases, 98% of Williams syndrome cases and 91% of patients with Smith Magenis. Details of the research are to be presented today at the British Association for the Advancement of Science meeting in York.

"Some specialists will be able to make a diagnosis as soon as a patient comes through the door, but some of these conditions are very rare, and it can take a lot of experience to recognise them. Some doctors will only have read about cases in journal papers," Dr Hammond said.

"Doctors can use this to focus in on the likely disorder before beginning genetic tests, which can cost £500 to £1,000 each, so the children and the parents should get a cheaper, quicker diagnosis."

Dr Hammond's team has created software that can morph from a healthy face to a face with any one of the 10 different genetic disorders they have so far studied. In January, the system will be used for the first time to teach doctors in Cambridge how to identify the disorders.

The technique is unlikely to be available across the country soon, because the 3D digital cameras needed typically cost £20,000. Of the 15 or so British hospitals that have the equipment already, most are in maxillofacial centres, where they are used to assess facial surgery operations.