Publicist Simon Jones confirmed the Little Mix star’s absence from Saturday night’s TV talent show final on Twitter.
“Jesy is unwell and will not be appearing on tonight’s final of Little Mix The Search,” he wrote on social media. “She will also not be hosting or performing at tomorrow’s MTV EMAs.”
It is not known what is wrong with Nelson or how unwell she is.
Fans took to social media to offer their support for the Holiday singer.
Singer Jack Remmington said: “Sending loads of love to Jesy. It’s not easy to be unwell in the public eye but we all love her so much and hope she’s back to feeling like herself again soon.”
The BBC One talent show Little Mix The Search has seen six bands compete for a place on the Little Mix 2021 tour.
The band – Nelson, Perrie Edwards, Jade Thirlwall, and Leigh-Anne Pinnock – were not among those who tested positive.
Little Mix formed on The X Factor in 2011 and have become one of the country’s biggest-selling girl bands.
They left Simon Cowell’s Syco Music for the RCA label in November 2018.
Cowell later launched another version of The X Factor, called The Band, to rival the girl group’s The Search.
Additional reporting by PA Media.