In a huge surprise, frontman Adam Gontier has left Canadian rock group Three Days Grace and been replaced by Matt Walst from My Darkest Days -- at least for the group's upcoming co-headlining tour with Shinedown.

The band cited a "non life threatening health issue" for Gontier's decision, while in an open letter to fans posted online the singer, acknowledging struggles with drugs and addiction, promised that "I am sober. That chapter in my life is over" and explained that "after twenty years of being part of an ever evolving band, I have been inspired by life to move on and to continue to evolve on my own terms" without addressing any specific reasons for leaving.

In Three Days Grace's statement, bassist Brad Walst says that the group was "surprised and distressed by Adam's sudden departure. We are however very excited for this upcoming tour and to have my brother Matt stepping in as lead singer."

The group posted a video Matt Walst rehearsing with Three Days Grace, while the new singer posted a message on Facebook headlined "What A Crazy Week," including the departure of guitarist Sal Costa from My Darkest Days and Walst's decision to help out Three Days Grace:

"It has always been a childhood dream to share the stage with my brother and I get that opportunity on this upcoming tour. Adam is an amazing Singer, Performer and Role-model and all I can do is try and fill some enormous shoes. I wish him well in his recovery and hope to see him grace the stage again soon."

Matt Walst added that My Darkest Days "is stronger than ever" and will continue after he finishes with Three Days Grace.

The tour with Shinedown begins February 1 in Moline, Illinois, and continues through the end of March.

Three Days Grace's latest album, "Transit of Venus," debuted at No. 4 on the Billboard 200 chart in October, launching the No. 1 Mainstream Rock Songs hit "Chalk Outline."

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