Chris Tarrant will be quizzing former Got Talent finalists including Paul Potts and Stavros Flatley in the famous hot seat.
Other reality TV stars taking part include teenage singer Shaheen Jafargholi and dance duo Twist & Pulse.
"We're well practiced as a team and we've been writing questions for each other in preparation. We've very high hopes of hitting the million!" said Twist Pulse.
Stavros Flatley's Demetrios Demetriou said: "We're very lucky to be in the position to take part in the show and help a great cause, I just need to become more decisive as I'm terrible at making decisions. Lagi will no doubt keep me in check."
Who Wants to Be a Millionaire: Britain's Got Talent Special airs on Sunday, May 6 on ITV1.