The discovery was made during research into the impact on asthmatics of a common environmental mould, Aspergillus fumigatus, usually found in soil and compost heaps.
Professor Andy Wardlaw, from the University of Leicester, said: "Asthma is a very common condition where the breathing tubes (bronchi) can go into spasm, making it difficult to breathe.
"Around a fifth of adults with severe asthma, which they have had for a long time, get permanent (fixed) narrowing of their bronchi. It is known that A. fumigatus can grow in the lungs of some people with asthma and mould allergy, which can cause severe lung damage.
"This problem is thought to only affect a very small number of people with asthma.
"However, about half of people with severe asthma have evidence of allergy to moulds like A. fumigatus."
Researchers at the Institute for Lung Health at the university and Glenfield Hospital carried out a study funded by the Midlands Asthma and Allergy Research Association and European Regional Development Fund.
They looked at whether the problem of A. fumigatus growing in the lungs is more common than previously thought, and whether this could explain the fixed narrowing of the airways in some people with asthma.
"Our study showed that six out of ten people with asthma who were allergic to A. fumigatus grew the mould from their sputum," Prof Wardlaw added.
"Our research concluded that it is possible that fixed narrowing of breathing tubes in many people with asthma could be caused by A. fumigatus growing in their lungs. Treating individuals from whom A. fumigatus is detected with antibiotics against the mould may prevent fixed narrowing of the airways."