JLS fans burst into tears after discovering their heroes had pulled out of T4 On The Beach.
The boyband cancelled their opening set citing production problems and blamed the lack of a sound check and time to rehearse a set especially created for the festival.
Singer Marvin Humes, 26, posted on the band's Facebook page: "We are SO sorry that we are not performing. It's something that was beyond our control.
"As a four-piece live singing band there is no way we can do such a huge show without a sound check or camera rehearsal.
"We are extremely disappointed to let you all down. All of our fans know us and know that when we perform we go 100% and are 100% happy that we can give you the best, there's no way that we would have done this today and our fans deserve only the best."
JLS - Humes, Aston Merrygold, Oritse Williams and JB Gill - rose to fame on X Factor finishing runners-up to 2008 winner Alexandra Burke.
Jessie J had already withdrawn on medical grounds after breaking her foot.
In a message posted on the T4 on The Beach website, the singer told her fans: "I was so excited about the summer and I'm as upset as you all are. But unfortunately these things happen. I have to remain positive and I promise I will make it up later in the year."
However, the disappointment of JLS and Jesse J pulling out did not stop around 50,000 music lovers basking in the glorious sunshine at Weston-super-Mare, Somerset.
The sold-out event featured acts including N-Dubz, The Wanted, Girls Aloud star Nicola Roberts, Olly Murs, Scouting for Girls and the Sugababes.