The singer's one-sentence statement to The Associated Press does not criticize the show, as her brothers Jermaine and Randy have done, but cites the court case as her reason for not attending.
"Because of the trial, the timing of this tribute to our brother would be too difficult for me," her statement reads.
Her statement was released on what would have been her brother's 53rd birthday.
The show is scheduled for Oct. 8 in Cardiff, Wales, and the singer's mother, Katherine, and some of her children are scheduled to attend.
Organizers of "Michael Forever - The Tribute Concert" have struggled to line up top-name acts and have been criticized by fans and the late singer's estate for multiple reasons, including its costly tickets, timing and remote location.
Fan groups from around the world have called on organizers to cancel the show.
Juliette Harris, a spokeswoman for promoter Global Event Live, had no immediate comment on Janet Jackson's statement Monday.
Christina Aguilera, Smokey Robinson, and Cee Lo Green are the top-names listed as performers so far. Organizers had to rescind an offer to the band Kiss after fans and Michael Jackson's estate noted that singer-bassist Gene Simmons had harshly criticized the pop singer over the years.
Opening statements in the involuntary manslaughter trial of Dr. Conrad Murray are scheduled to begin on Sept. 27. Murray, who has pleaded not guilty, could face up to four years in prison if convicted.
Authorities allege he gave Michael Jackson a lethal dose of the anesthetic propofol and other sedatives in the bedroom of his rented mansion on June 25, 2009.
Jermaine and Randy Jackson issued a statement on the day the concert was announced, saying Murray's trial took precedence over other events.
"There will come a time and place for an amazing and deserving tribute to Michael," the brothers' statement read. "But we feel that the most important tribute we can give to our brother at this time is to seek justice in his name."
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