"While we wholeheartedly support the spirit of a tribute that honors our brother, we find it impossible to support an event that is due to take place during the criminal trial surrounding Michael's death," Jermaine and Randy Jackson said.
Dr. Conrad Murray's involuntary manslaughter trial begins in September and is expected to continue through October.
Members of the Jackson family taking the stage October 8 include several of Michael Jackson's brothers, "the next generation of Jacksons," and 3T, which consists of Tito Jackson's three sons, the promoters said.
Alternative rock band Alien Ant Farm and British R&B singer Craig David are also on the bill.
"This concert will unite various generations and musical genres, reminding everyone of just how amazing a talent Michael was," Global Live Events executive Paul Ring said.
Fans can "register their interest for tickets" online at http://www.michaelforevertribute.com/ through Thursday, the promoter said. A lottery will determine which fans will be invited to purchase tickets, it said.
The concert in the Wales Millennium Stadium in Cardiff, which seats 75,000 people, would be at least four hours long, Global Live Events CEO Chris Hunt said.
A spokesman for Michael Jackson's estate said last month that the promoters never sought or obtained permission from the estate's executors to use the late pop singer's name or image.
"The estate has not been contacted and is not involved with Global Live Events in the UK tribute concert announced today," spokesman Jim Bates told CNN at the time.
According to CNN KISS are signed up to play in a tribute concert to Michael Jackson at the Millennium in Wales! Now the weird bit, they are on the same line up as JLS, Craig David and Christina Aguilera!!