"EastEnders" legend Leslie Grantham was working on a film about the Kray twins just days before he fell ill.
The 71-year-old actor - who played "Dirty" Den Watts on the BBC One soap - is "fighting for his life" as he battles a mystery illness after returning to the UK from Bulgaria, where he has been living for years, and he was admitted to hospital shortly after he finished filming "The Krays: Dead Man Walking".
A friend told the Daily Star newspaper: "He was happy to be home in England and had hoped this was going to put him back in the spotlight.
"He was ready to return to acting with a bang but it seems like he's been cursed."
The former Albert Square regular - who plays policeman Nipper Read, who arrested the infamous London gangsters Ronnie and Reggie Kray, in 1968 - is being closely monitored by doctors after falling gravely ill, but the cast and crew of "EastEnders" are praying for the actor.
Lorraine Stanley - who portrays Karen Taylor - said the show's stars "wish him all the best", and her on-screen son Zack Morris (Keegan Baker) is hoping the "EastEnders" legend pulls through.
He said: "I hope he gets well soon.
"We all wish him a speedy recovery."
Leslie's agent has confirmed that a statement about the "poorly" actor's situation will be released "at an appropriate time".
A close friend said earlier this week: "Leslie is being looked after. Those who are close to him are hugely worried and are praying for his well-being."
The former "Paradise Club" star moved out of the UK following his split from his wife Jane in 2013, after 31 years of marriage, to Bulgaria after spending time in the country whilst filming soap "The English Neighbour" in 2010.
Leslie is responsible for one of the biggest moments in "EastEnders" history when Den delivered divorce papers to his wife Angie Watts (Anita Dobson) on the soap's Christmas Day episode in 1986.
More than 30 million people - over half of the British population - tuned in to watch the momentous scene.
Den was famously shot dead just three years later and his on-screen daughter Sharon Watts (Letitia Dean) identified his body.
But "EastEnders" bosses brought his character back from the dead in 2003, when he was reunited with Sharon with the line: "Hello, princess."
Leslie's two stints as Den - from 1985 to 1989 and 2003 to 2005 - were made even more believable by his own criminal past.
Before becoming an actor he spent a decade in prison from 1967 for the murder of a German taxi driver. While behind bars, he learned how to act and performed in plays for his inmates.