TMZ broke the story ... Lauren lost her voice and could not finish rehearsing for tonight's finale.
Producers have brought Haley Reinhart to the set and she is currently in wardrobe. If Lauren can't compete, Haley will square off against Scotty McCreery for the title.
We're told Haley just finished rehearsing, and if she does compete tonight she'll sing 3 songs -- "House of the Rising Sun," "I Who Have Nothing" and "Bennie and the Jets."
