Asthma UK urged all patients with the condition to do what they can to prevent colds and flu and to ensure their medications are fully stocked.
The charity said a previous study found that 81% of sufferers say cold and flu viruses can make their condition worse.
This could mean that as many as 4.3 million asthma patients across the UK are at heightened risk of an attack, the charity estimated.
It’s worth noting that most people with asthma are eligible for a free flu vaccine. However typical effectiveness of the jab is in the range of 40-60%, meaning people can still get flu.
Dr Andy Whittamore, Asthma UK’s clinical lead, said: “The flu outbreak this year is extremely concerning and needs to be taken seriously.
“As many as four in five people with asthma could be at an increased risk of life-threatening asthma attacks if they catch the virus.
“The best way for people with asthma to stay safe this winter is to make sure they take their asthma medicines as prescribed.
“This ensures their airways are less inflamed and sensitive, and will help them to resist the effects of the flu.
“Everyone with asthma should also make sure they are washing their hands regularly to prevent spreading viruses, and look after themselves by eating a varied, balanced diet, exercising regularly and getting enough sleep.”