Stewart, who has played PC Reg Hollis on the police drama since 1984, was rushed to St George's Hospital in Tooting, South London, after the incident yesterday.
He is believed to have slashed his wrists in his dressing room moments after being told that his contract would not be renewed.
The decision follows his week-long suspension last month after losing his temper during filming.
The actor alerted security at the studio in Merton, South London, before being taken to hospital, where he was kept in overnight.
An source told The Sun: "This was a desperate cry for help. It's extremely sad. He is in hospital and is being cared for."
The 52-year-old suffered an emotional breakdown last year after collapsing at his home. He later admitted that he had spent time in rehab but felt he was back on track, saying: "There is no way I would ever take my own life."