Antibiotics can increase vulnerability to flu and worsen symptoms, a study has found.

Flu defences in the lung were weakened in mice with healthy gut bacteria who were given antibiotics before being infected, according to research from the London-based Francis Crick Institute.

Researchers found that signals from gut bacteria keep antiviral genes in the lung lining active, helping maintain a first line of defence against flu.

One third of the mice survived when given antibiotics before becoming infected, compared to 80% who survived without the drugs.

Two days after infection, mice which had received antibiotics had five times more virus in their lungs, the study published in the scientific journal Cell Reports found.

As a result the immune response was stronger and more damaging, leading to more severe symptoms.

Dr Andreas Wack, who led the research, said: "We found that antibiotics can wipe out early flu resistance, adding further evidence that they should not be taken or prescribed lightly.

"Inappropriate use not only promotes antibiotic resistance and kills helpful gut bacteria, but may also leave us more vulnerable to viruses.

"This could be relevant not only in humans but also livestock animals, as many farms around the world use antibiotics to prevent the spread or occurrence of disease or infection.

"Further research in these environments is urgently needed to see whether this makes them more susceptible to viral infections."

The researchers found that cells lining the lungs were more important than immune cells in flu resistance at the early stages of infection.

They said: "They are the only place that the virus can multiply, so they are the key battleground in the fight against flu.

"Gut bacteria send a signal that keeps the cells lining the lung prepared, preventing the virus from multiplying so quickly."