They are recruiting 220 asthmatic volunteers for the six-week study which initially involves a low-salt diet.

Then they will be put into two groups, one will continue with their diet while the other will eat food containing the UK national average amount of salt.

Recent studies have shown that asthmatic people with a high salt intake can benefit from reducing it.

The project, funded with �165,000 from the charity Asthma UK, will also test whether restricting dietary salt is a long-term option.

In the UK, 5.2 million people have the condition and 50% of these have severe symptoms that have a major impact on their daily lives.

Clinical senior lecturer Dr Andrew Fogarty said: "Our trial will look at the role of sodium restriction in controlling asthma.

"If this proves beneficial then a low salt diet is an easy and simple way to help improve asthma symptoms for all sufferers."

According to Asthma UK, asthma causes more than 69,000 hospital admissions a year and costs the NHS �889m a year to treat. On average, 1,400 people a year die from the condition.