Collins has refused to use his mother's battle with breast cancer as a sob story during the competition, not talking about his feelings for Tracy during VT footage.
However, his Take That star mentor revealed in a final press conference this morning at Wembley Arena that their performance together on Saturday night would have particular significance for the 23-year-old Liverpudlian.
"On Saturday night we're singing the Billy Joel classic together 'Always a Woman'. I know that Marcus is dedicating the song to his mum on Saturday, which is amazing," said Barlow. "I know she's been really important in his life. It's going to be an emotional evening."
Watch Gary Barlow talking about the duet at the conference this morning:
Praising Collins's growth during the X Factor live shows, Barlow added: "The way he has grown through the competition is amazing. He's been an absolute pleasure to work with."
Barlow also claimed that yesterday's homecoming trip to Liverpool with Collins had made the strain and pressure of X Factor "all worthwhile".
"It was an amazing homecoming yesterday at Marcus's house with a baby on my knee and his nanna and granddad giving me stick on the other side," he said. "The whole of Liverpool were out last night, it was a brilliant show."
The X Factor final kicks off on Saturday (December 10) at 8pm on ITV1.
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