The Eastenders villain, 71, who played “Dirty” Den Watts, has returned to the UK and is now being closely monitored by docs.
Leslie has been secretly visited by close pals in the industry who are now fearing the worst.
A close friend said: “Leslie is being looked after. Those who are close to him are hugely worried and are praying for his well-being.”
The London-born actor moved out of the UK following his split from his wife Jane following 31 years of marriage.
He spent time in Bulgaria filming soap The English Neighbour in 2010.
A spokeswoman for Leslie confirmed to The Sun: “It’s true that Leslie is very poorly.”
The London-born actor was famously brought back from the dead by EastEnders bosses in 2003 when his character Den was reunited with on-screen daughter Sharon Watts in one of the soap’s most shocking ever storylines.
Fourteen years earlier, he was supposedly shot dead by a canal during an attempted bid to leave the country and Sharon even identified his body.
Leslie’s portrayal as an EastEnders villain was made more believable by the fact he spent a decade behind bars from 1967 for the murder of a German taxi driver.
During his stint in prison, which was served in various British jails, he learned how to act and performed in plays for the benefit of inmates and invited members of the public.
He landed a role in EastEnders seven years after his release.
In 1986 the soap’s Christmas Day episode, which saw him hand divorce papers to alcoholic first wife Angie, was watched by 30.15 million TV viewers.
However, his second spell in Walford was overshadowed by his part in a newspaper sting.