Singer Bryan Ferry has had to cancel his appearance at the Summer Pops festival in Gwynedd because he has lost his voice.
Promoters had a call early on Sunday to say Bryan Ferry had cancelled
Four thousand fans had been expected to attend Sunday's concert on the Faenol Estate near Bangor.
Organisers said tickets would be valid for the Will Young show on Monday - the highlight of the four day festival.
Ferry, the lead singer of the 1970s rock band Roxy Music, has issued a personal apology to all his fans.
The show's promoters CMP Entertainment said they were "heartbroken" when they received the telephone call at 0130 BST on Sunday saying Ferry had had to pull out.
The star had been in action in Oxford on Saturday after cancelling several other shows following a recent bout of illness but then suffered more throat problems.
"It stopped us in our tracks," said a spokeswoman.
"The atmosphere had been really building with people rocking in the aisles and going really crazy over the first couple of days."
"But we can't control the weather and unfortunately nor can we control what happens to the performers".
All those with tickets for the Ferry concert will now be accommodated - if they want - in Monday's performance by the ex-Pop Idol winner Will Young. Otherwise fans can get their money back.
Organisers said there was no problem finding room for the extra fans - around 10,000 are expected on Monday night - as the extra numbers can be put on the lawn area.
Hugh Edwin Jones from Gwynedd Events Commission said he was disappointed about Sunday's cancellation, but added he was happy with Summer Pops so far and confident about its future.
"It was worth fighting for it [the festival] and I hope that the festival will build to be an important event.
In May, Gwynedd Council stepped in after the promoters of Bryn Terfel's Faenol Festival - held in August and now in its fifth year - said it would be threatened by the Summer Pops event.
The council offered to guarantee the opera star's event against any losses using European Union Objective One money.
Jazz performer Jamie Cullum and Bryn Terfel himself are scheduled to perform at that August Bank Holiday event.
Another veteran pop star Sir Cliff Richard performed at Cooper's Field in Cardiff on Sunday.