Alex Zanardi has been transferred to intensive care in Milan amid concerns about his unstable condition, following the recent move to a clinic.
The Italian suffered serious head injuries in a handbike accident last month and was in an induced coma for several weeks as doctors battled to save his life.
His recovery appeared to be moving forwards following the decision to move him away from the intensive care unit in Siena where he had been treated to a clinic in the province of Lecco.
But doctors issued a note on Friday to say that amid worries about his condition, it was felt better that he be returned to intensive care.
In a note signed by Claudio Zanado, the health director of the Valduce Hospital in Como where he has been since the start of this week, it was made clear that the call was made after a joint decision by senior medical staff.
"Today, in the face of the instability of the clinical conditions of patient Alex Zanardi, and after appropriate consultations with Dr. Franco Molteni, head of the Department of Specialist Rehabilitation Villa Beretta where the patient had been hospitalised since 21 July, and the reference specialists, the transfer of the athlete was ordered," he wrote.
"Adequate means and adequate assistance were used at the Intensive Care Department of the San Raffaele Hospital in Milan. As of now, it is specified that no further information on the case will be released."
Zanardi underwent a series of neurological operations at the Santa Maria alle Scotte hospital in Siena following his collision with a lorry during the Obiettivo Tricolore handbike racing event on Highway 146 near Siena.
The Italian is a leading Paralympic athlete, having won four gold medals on his handbike in the 2012 and 2016 games.
The former grand prix driver won the CART championship twice prior to an accident at the Lausitzring in 2001 that resulted in both his legs being amputated.