Sir Elton John has cancelled a headline performance at London's Hyde Park after being diagnosed with appendicitis.
The star had been due to perform at the concert on Friday night, topping a bill which also features Elvis Costello and Ray Davies.
He called in doctors after being taken ill during his European tour.
Sir Elton, 66, had undergone a series of scans and blood tests with medics in the UK and is now undergoing am intensive course of antibiotics.
A statement on behalf of the singer said he had been diagnosed with an "appendix abscess surrounding retrocaecal appendicitis".
He is expected to undergo surgery in the UK in the next few weeks when doctors are sure they have reduced toxins around the inflamed appendix sufficiently.
Doctors advised him to cancel all forthcoming shows from his current tour, including the Hyde Park show which is part of the Barclaycard Presents British Summer Time Festival.
The singer's spokesman said Sir Elton will not return to touring until fully recovered from the surgery and the damage caused by the abscess. He went on: "Elton is incredibly disappointed to postpone these tour dates. To know that he made such super-human efforts and continued to perform to thousands through his illness only confirms his dedication to his European fans.
"He is eager to be back on top form and return to play the remaining shows starting in early September 2013."
Sir Elton was confirmed only today as one of the stars due to play the iTunes Festival at London's Roundhouse in September.
UPDATE: AEG Live and The Royal Parks have confirmed that the show will go on in Hyde Park with the remaining line-up for free. Existing ticket holders will receive a full refund.
Davies and Costello will be joined by Gabriele Aplin, Nick Lowe and many more across the park on Friday.
Existing ticket holders for Friday 12th July will automatically receive a full refund - the full value of their original ticket orders will be credited to the original payment method within 7 working days. And by bringing their existing tickets along to Barclaycard presents British Summer Time, Hyde Park on Friday 12th July only, they will be given free access.
AEG Live is also making further tickets available for Friday 12th July - all fans need to do is log onto www.bst-hydepark.com/12julyupdate and print out a ticket at home to bring with them to Hyde Park. Tickets are available on a first come, first served basis until capacity.