The 71-year-old star is under medical supervision after reportedly returning to the UK from Bulgaria, where he had been living, for treatment.
Zack Morris, who plays Keegan Baker in the BBC soap, told the Daily Star: “I hope he gets well soon.
“We all wish him speedy recovery,” he added.
EastEnders actress Lorraine Stanley, who plays Keegan’s mother Karen Taylor, also said the whole cast “wish him the best”.
The Sun previously reported Leslie was “fighting for his life” and that close friends in the industry were “fearing the worst”.
“Leslie is being looked after,” a source told the publication.
“Those who are close to him are hugely worried and are praying for his well-being.”
A representative for Leslie told Express.co.uk: “Leslie Grantham remains poorly and is being taken care of by medical staff.”
They also ask that the “privacy of Leslie’s family and friends” be respected, adding: “A statement will be issues at the appropriate time.”
Leslie moved to Bulgaria after his marriage to Jane, his wife of 31 years, broke down.
He had previously spent time in the eastern European country while filming The English Neighbour back in 2010.
The actor left Walford for good 14 years ago but his legacy as the infamous villain
Dirty Den famously drew in over 30 million viewers on Christmas Day 1986 when he handed over divorce papers to wife Angie (played by Anita Dobson).
Although he was supposedly killed off in 1989, he made a shock comeback when he returned to Albert Square in 2003.
It was his return which spawned the now iconic line: “Hello Princess.”
Dirty Den met his lasting demise on the show in 2005 when Chrissie Watts (Tracy-Ann Oberman) whacked him over the head with an iron.
She then buried him under the cellar of his beloved Queen Vic.
In recent years, Leslie turned his hand to writing and produced his first children’s book, Jack Bates and the Wizard’s spell.
At the time, he told the Mirror: “Life isn’t a straight line, it’s like traveling the motorway.
“Every now and then, you have to take a diversion.
“Unfortunately, some of my diversions have been have been quite catastrophic,” he added. “But I’m safe in the knowledge that what I do now is good.”