Gray, who played Sue Ellen Ewing on "Dallas", had confirmed the casting news last month.
But now Collins, 83, says TV bosses at Channel 4 in the UK came to her first, reported Contactmusic.
"I was just asked to be in Hollyoaks but I'm not doing it. You know who is doing it? Linda Gray," she said.
Collins said she was too busy for "Hollyoaks", thanks to a new British stage show and an upcoming movie project, adding, "I wouldn't have the time for anything else.