Alex Zanardi was in a "very serious" condition with a head injury after undergoing surgery following a road accident in Italy, doctors said.
The former Formula One driver, who in 2001 suffered a horrifying motorsport accident that resulted in both of his legs being amputated above the knee, was widely reported to have collided with a large vehicle on Friday while riding his handbike.
Zanardi made a heart-warming sporting comeback as a Paralympian who won gold medals for Italy at London 2012 and Rio 2016.
The motorsport and Paralympic worlds were hoping he would recover from the latest trauma, with Damon Hill and Mario Andretti among those expressing concern for the 53-year-old.
The Senese-University Hospital in Siena issued a statement that said Zanardi was being treated at its Santa Maria alle Scotte polyclinic, having been airlifted from the scene of the accident.
It said his injury was "severe", and a later statement said: "Regarding the clinical conditions of Alex Zanardi, hospitalised in very serious conditions at the Santa Maria alle Scotte polyclinic due to a road accident that occurred in the province of Siena, the Aou Senese Health Department informs that the neurosurgical and maxillofacial intervention to which the athlete was subjected, due to the severe head injury, started shortly after 19:00 and ended shortly before 22:00.
"The patient was then transferred to intensive care, with a reserved prognosis. His health conditions are very serious."
As well as hand-cycling, Zanardi has competed in the World Touring Car Championship since his 2001 crash.
In Formula One, he drove for the Jordan, Lotus, Minardi and Williams teams during the 1990s.
Former Formula One world champion Hill, a contemporary of Zanardi, wrote on Twitter: "Shocked and praying he is OK. To him and his family and friends and millions of fans and supporters, we're praying for you Alex."
Andretti, who won the F1 world championship in 1978 and also drove with great success in IndyCar, wrote: "I am so anxious and frightened about Alex Zanardi that I'm holding my breath. I am his fan. I am his friend. Please do what I'm doing and pray pray for for this wonderful man."