The London Ambulance Service was called to her home in north London just after 4:00pm local time but medics were unable to save her life, saying she was "beyond help," according to Sky News.
"On arrival, officers found the body of a 27-year-old female who was pronounced dead at the scene," a police statement said.
"Inquiries continue as to the circumstances of the death. At this early stage it is being treated as unexplained. There have been no arrests in connection with this incident," Police Superintendent Raj Kohli told reporters outside her home.
"I am aware of reports suggesting that the death was a result of a suspected drugs overdose, but I would like to re-emphasize that no postmortem examination has yet taken place and it would be inappropriate to speculate on the cause of death," he added.
Winehouse's body was removed from her apartment late Saturday, where grieving fans gathered to leave tributes and flowers. An autopsy is expected to be conducted either Sunday or Monday.
Winehouse had last been seen publicly Wednesday at the iTunes festival in North London.
While she didn't sing, Winehouse staggered on stage and begged the crowd to buy her protege, 15-year-old Dianne Bromfield’s new album.
Last month she cancelled her European tour following a disastrous performance in Belgrade, Serbia where she gave an incoherent performance appearing disoriented and drunk.
Winehouse had a long history of substance abuse, including not only alcohol but heroin and cocaine. As recently as June she had to cancel a planned 12-stop European tour after appearing drunk and struggling to remember her own lyrics at the tour's first stop in Belgrade, Serbia.
Prior to beginning the tour, she spent a week at the Priory Clinic in London, which offers treatment for addiction problems.
"Everyone involved with Amy is shocked and devastated. Our thoughts are with her family and friends," a spokesman for the singer said.
Her father, Mitch Winehouse, a part-time singer himself, was en route back to London from New York Saturday, where he had traveled to perform at Manhattan's Blue Note Jazz Club.