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."
The band - which also includes Marcus Mumford, Ben Lovett and Winston Marshall - announced they are postponing three upcoming US shows this week, but hope to continue with the rest of their summer shows.
One worry is Ted's health scare could jeopardise the band's headlining slot at Glastonbury festival later this month - one of their biggest concerts to date, watched by an estimated 170,000 people on site and a TV audience of millions - but the group are adamant he will make a quick recovery.
They continued: "Sadly we have to postpone three upcoming [US] headline shows planned for this week in Dallas on Tuesday, Woodlands on Wednesday, and New Orleans on Thursday.
"All dates will be rescheduled and will be announced as soon as we can; all tickets for this week's events will be honoured at their rescheduled dates.
"We have no plans to cancel or postpone any other appearances along this current tour. We're all wishing Ted a speedy recovery."
- Bang! Showbiz