Previously, only smoking cessation has been shown to slow progression of the disease.

Findings from the 'Towards a revolution in COPD health' (TORCH) study suggest that treatment with a salmeterol/fluticasone propionate inhaler slows the rate of lung function decline compared with placebo.

TORCH has shown that combination treatment reduces the number of COPD flare-ups but has no effect on mortality rate.

The study included 6,112 patients, aged an average of 65, who had moderate to severe COPD. Participants were randomly assigned to receive fluticasone propionate 500mg daily, salmeterol 50mg twice daily, both or a placebo for three years.

Post-bronchodilator forced expiratory volume in one second (FEV1) was assessed at baseline and every six months.

Those taking the combination inhaler had a 92ml smaller decline in FEV1 over the study period than those given placebo, it was announced at the American Thoracic Society (ATS) conference in California, US, this week.

The combination inhaler led to an average 50ml smaller decrease in FEV1 than those given salmeterol alone, and a 44ml smaller decrease than those given fluticasone propionate alone.

One of the researchers, Dr Bart Celli, chief of pulmonary and critical care at St Elizabeth's Medical Centre in Boston, said: 'It shows that pharmacotherapy impacts outcome.

'Clinicians will likely use the combination in milder forms of COPD compared with what has been the standard.'

Why combined inhalers slow the progression of COPD is still uncertain, he added.

'There are several possibilities including a decrease in inflammation, persistent airway dilatation and other mechanisms.'