The four piece - who have appeared in countless West End musicals - have created The Cardinals.
Forster took to his Twitter to make the announcement this week following his solo concert Me, Myself and Musicals at Theatre Royal Haymarket.
He wrote: "So I’m in a group. With these amazing people.
"We are “The Cardinals” #thecardinals @music_cardinals reimagining songs we love. @CarrieHFletcher @CelindeSchoen @raminkarimloo"
So I’m in a group. With these amazing people. We are “The Cardinals” #thecardinals @music_cardinals reimagining songs we love. @CarrieHFletcher @CelindeSchoen @raminkarimloo pic.twitter.com/3pQYawloTJ
— Ben Forster (@thebenforster) June 24, 2019
Karimloo shared to his Instagram more details: "Was incredible to finally put @thecardinalsmusicofficial on it’s feet last at @thebenforster ‘s unparalleled solo show. It’s been exciting starting to play around with some ideas together and create.
"Let’s see what our schedules allow over the next coming months. But I certainly don’t need any excuse to spend time with Ben, @celindeschoenmaker and @carriehopefletcher Making music is an unbelievable bonus. Thanks for the support already. 🖤"
And Fletcher added: "Last night, @raminkarimloo, @celindeschoenmaker and I sang with @thebenforster at his concert but not individually. Oh no. We sang together as the group we’ve been creating since the beginning of this year. We’re four very busy people with careers of our own so we wanted something that we could have fun with, something that’s our own, in all the in-betweens.
"What happens next, we don’t know but we know it’s something exciting. I do know that I couldn’t feel luckier to be on stage with four more talented, gorgeous people and to finally introduce to you, The Cardinals."