The rocker reached out to the Girls Aloud singer and asked her to perform at Roger Daltrey's fundraising concerts in aid of The Teenage Cancer Trust, which he is organising while The Who frontman is on tour.
Paul Weller, Kasabian and Primal Scream are among the acts lined up for the London gigs, and Walsh admits she was surprised to receive an invite from the former Oasis rocker.
She tells Britain's The Sun newspaper, "The Girls Aloud tour is around the same time so we couldn't make it work. I'm gutted (disappointed) that we can't do it.
"He called me up out of the blue. It's great that he's involved and is arranging it. It was exciting to speak to him. I was like, 'Really?' He said I was his favourite (Girls Aloud member) and (bandmate) Sarah (Harding) wasn't very happy about that."