Children can find it difficult to use peak flow meters

It allows the condition to be checked while they breath normally - unlike existing peak flow meters, which ask patients to blow as hard as possible.

Peak flow meters do not work for children under seven, and can also be difficult for some adults to use.

Asthma Alert was devised by scientists at Brunel University, who say it is so simple even toddlers will be able to use it.

There is no single test for asthma and this can make diagnosis difficult, especially in young children National Asthma Campaign spokeswoman

They say the hand-held device, which is still in development, could be commercially available within 18 months.

It uses a technique known as capnography to measure the rate of change in the concentration of exhaled carbon dioxide against time with normal breathing.

Its makers say it is easy for patients to interpret results and then alter their medication if necessary.

More than 50m people are being treated for asthma across the world, and it kills more than 1,500 people in the UK each year.

One in eight children have the condition.

Diagnosis difficulties

Dr Deryk Williams, who worked on the device, said: "It's simplicity, reliability and low cost will make it ideal for patients to monitor their own asthma and be in control of the disease.

"It will also be of immense benefit to GPs, who will now be able to diagnose asthma far more accurately."

Asthma Alert Ltd recently won the Smart Award from the Department of Trade and Industry's Small Business Service, which encourages innovation.

A spokeswoman for the National Asthma Campaign told BBC News Online: "There is no single test for asthma and this can make diagnosis difficult, especially in young children.