The 50-year-old is understood to have collapsed at his north London home. Two emergency ambulances were sent, the London Ambulance service confirmed.
George Michael reportedly collapsed at home last week
“We were called just before 8am on May 22," their spokesperson told the Mirror.
"We sent two ambulance crews and staff treated one patient, a man, who was then taken to hospital."
Witnesses said the vehicles left four hours later, taking George with them.
On Wednesday night, the singer's agent told the Sun newspaper: "George Michael was in hospital for routine tests but there is nothing further to say."
Fans have been concerned about the freedom singer's wellbeing in recent years after a run of health problems and the accident he was involved in last year.
Fears have been raised for the singer's health in recent years
In May 2013 George was airlifted to hospital and treated for head trauma after he seemingly fell from a moving car on the M1 motorway.
The incident happened 18 months after the star's near-death experience with pneumonia in 2011.
George said that for three weeks it was "touch and go" as to whether he would pull through. He even had to learn to walk again after his muscles wasted away during his recovery.
In an interview for Radio 2, the Freedom singer admitted: "It's like I just dodged a bullet," also revealing that he has no memory of the time when his life hung in the balance.