U2 have pulled out of the Glastonbury Festival and postponed their US tour after Bono had emergency surgery to save him from possible paralysis.
The singer, 50, had a back operation on Friday after suffering an injury while training in preparation for the tour.
Neurosurgeon Joerg Tonn said: "The surgery was the only course of treatment for full recovery and to avoid further paralysis."
Bono, who must recuperate for the next two months, said he was "heartbroken".
They were due to headline Glastonbury on 25 June, in the middle of their 16-date North American tour, which was booked for June and July.
Prof Tonn told the Reuters news agency Bono had been "in severe pain with partial paralysis of the lower leg".
The frontman is "now much better with complete recovery of his motor-deficit", he continued.
"His prognosis is excellent but to obtain a sustainable result he must now enter a period of rehabilitation."
Dr Muller Wohlfahrt, who also treated the star in Munich, Germany, explained: "Bono suffered severe compression of the sciatic nerve.
"On review of his MRI scan, I realised there was a serious tear in the ligament and a herniated disc, and that conservative treatment would not suffice.
"We are treating Bono as we would treat any of our athletes and, while the surgery has gone very well, the coming weeks are crucial for a return to full health."
His rehabilitation will begin in the next few days, with the intensity increasing over the next eight weeks.
The band's manager Paul McGuinness said the group were "devastated" at postponing the concerts.
"For a performer who lives to be on stage, this is more than a blow," he said.
"His concerns about more than a million ticket buyers whose plans have been turned upside down, we all share, but the most important thing right now is that Bono makes a full recovery."
Glastonbury had been attempting to book U2 for many years and their appearance was expected to be a highlight of the event's 40th anniversary.
The band said they had written a new song for the occasion.
"I'm heartbroken," he said. "We really wanted to be there to do something really special."
Bono phoned festival organiser Michael Eavis on Tuesday morning to break the news.
Mr Eavis said: "It was obvious from our telephone conversation that U2 are hugely disappointed. Clearly they were looking forward to playing the Pyramid Stage as much as we were looking forward to watching them."
He declined to comment on a possible replacement. "Instead, we would simply like to send Bono our very best wishes for a full and speedy recovery," Mr Eavis added.
Dizzee Rascal, who is below U2 on the Pyramid Stage bill on Friday 25 June, could now take the headline slot. Festival favourites Coldplay have also been mentioned as possible replacements.
The festival has had to cope with headliners pulling out in the past.
In 1995, The Stone Roses cancelled when guitarist John Squire broke his collarbone in a cycling accident. Pulp were brought in as last-minute replacements.
In 2005, Kylie Minogue withdrew following her diagnosis with breast cancer. Basement Jaxx were promoted to fill in.
U2's North American dates will be rescheduled.
Their 360 Degree tour earned the band about $109m (£75.3m) in 2009, making it the most profitable tour of the year, according to Billboard magazine.