British folk stars Mumford & Sons have been forced to postpone three upcoming shows after one of their number developed a clot on his brain.
Bassist Ted Dwane, 28, is currently receiving emergency treatment in hospital in America.
The quartet is due to headline at Glastonbury on Sunday 30 June and the news of Dwane's illness puts their performance in jeopardy.
A statement from the band said the musician had been "feeling unwell for a few days".
It said: "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."
The four-piece band shot to fame with their debut album, Sigh No More, which was shortlisted for the 2010 Mercury Prize. It's follow-up, Babel, was one of the biggest-selling albums of 2012 in both the UK and US, topping both the Billboard and UK Official Album charts.
The band has been forced to cancel gigs in Dallas tonight, the Woodlands, Texas on Wednesday and New Orleans on Thursday.
They said the shows would be rescheduled as soon as possible. No mention was made of their impending Glastonbury slot.
The Somerset festival's lineup has previously had to be reshuffled. Back in 2010 U2 had to cancel when singer Bono underwent surgery for a compressed sciatic nerve.
A Glastonbury source told The Mirror: "While Mumford & Sons still plan to perform at Glastonbury, organisers will surely be a little concerned about Ted's health and will be having a look at possible stand-ins should the worst come to the worst."