"We've just learned Michael Jackson has died," TMZ said.
"Michael suffered a cardiac arrest earlier this afternoon and paramedics were unable to revive him. We're told when paramedics arrived Jackson had no pulse and they never got a pulse back," the entertainment site said.
There was no official confirmation of the reported death and spokespersons for Jackson could not be reached for comment.
Earlier, the Los Angeles Times said that the singer had been rushed to a Los Angeles-area hospital by fire department paramedics who found him not breathing when they arrived at the singer's home.
The newspaper said paramedics performed cardiopulmonary resuscitation at the scene before taking him to the UCLA Medical Center hospital.
Jackson had been due to start a series of comeback concerts in London on July 13 running until March 2010. The singer, whose hits include "Thriller" and "Billie Jean," had been rehearsing in the Los Angeles area for the past two months.
The shows for the 50 London concerts sold out within hours of going on sale in March.
Jackson started out as a child star in the band "The Jackson 5" more than 40 years ago.
He has lived as a virtual recluse since his acquittal in 2005 on charges of child molestation.
There have been concerns about Jackson's health in recent years but the promoters of the London shows, AEG Live, said in March that Jackson had passed a 4-1/2 hour physical examination with independent doctors.
(Additional Reporting by Jill Serjeant; Writing by Frances Kerry, Editing by Jackie Frank)