Mumford & Sons' Ted Dwane has a blood clot on his brain.
The 28-year-old bassist was hospitalised yesterday and a scan revealed he will have to undergo emergency surgery.
A message posted on the band's official website said: "Ted has been feeling unwell for a few days, and yesterday he was taken to a hospital to receive emergency treatment.
"The scans revealed a blood clot on the surface of his brain that requires an operation.
"Ted is receiving excellent care and we are being assured that he will recover quickly from surgery."