Allen, 24, was due to appear at Benicassim festival at the weekend, but doctors told her that she was too ill to get on a plane and perform after coming down with a bout of gastroenteritis. She is very sorry to let fans down and hopes to be back to full strength as soon as possible," Contactmusic quoted a representative for Allen, who was replaced by rockers Maximo Park at the event, as having written on her MySpace.com blog.
The star took to her Twitter.com page to later express her regret at missing the festival.
"Wish I was at Benicassim, got a bug and been vomiting all day. Thank you to the lady who gave me a bag to vomit in at Gatwick (airport) this morning," she wrote.