Leanne Metcalf, Director of Research at Asthma UK, said:

"There is now a large body of evidence suggesting a link between female hormones, including the use of HRT, the development of asthma and its severity. However this is the first large-scale and long-term study to suggest that it is oestrogen-only HRT which significantly increases the risk.

"It's still too early to say exactly how the menopause or taking HRT affects asthma symptoms and who is likely to be affected. Therefore women who are intending to use HRT need to weigh up the broader health benefits of HRT with the possibility of an increased risk of asthma, especially those with a history of asthma or allergy in their family.

"We know that some women develop asthma symptoms during periods of hormonal change so it's important to speak to a doctor if you start to experience regular symptoms like breathlessness, cough or wheeze.

"Key stats:-
• Asthma is difficult to detect in older people as some of the symptoms – cough, wheeze and shortness of breath – are shared with other conditions such as chronic bronchitis, emphysema and heart disease

• More than a fifth of people with asthma are diagnosed after the age of 34."