Jesy Nelson is taking an "extended" break from Little Mix for private medical reasons.
The singer, 29, confirmed the news through a statement from the band's management.
Jesy has missed a number of public appearances in recent weeks including the final of Little Mix's BBC One talent show The Search and a performance at the MTV European Music Awards.
A statement from Little Mix's publicist Simon Jones said: "Jesy is having extended time off from Little Mix for private medical reasons.
"We will not be issuing any further comment currently and ask media to please respect her privacy at this time."
It is not known what is wrong with Jesy or how unwell she is.
Her hiatus comes shortly after the release of Confetti, Little Mix's sixth studio album and first since departing Simon Cowell's label Syco.
Little Mix - which also features Jade Thirlwall, Perrie Edwards and Leigh-Anne Pinnock - formed on The X Factor in 2011 and have become one of the country's biggest-selling girl bands.
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They left Syco Music for the RCA label in November 2018.
Simon Cowell later launched another version of The X Factor, called The Band, to rival the girl group's The Search.
