The 71-year-old actor has recently returned from Bulgaria where he has lived for several years.
Doctors are now said to be treating him, and close friends from the industry have been visiting his bedside, according to The Sun.
A spokeswoman told The Sun It’s true that Leslie is very poorly.
When contacted by Sky News, a representative for Grantham asked for the privacy of family and friends to be respected.
The actor played EastEnders’ most infamous villain, Dirty Den Watts, from 1985 to 1989 – making a surprise return to Albert Square 14 years later.
The 1986 Christmas Day episode – in which Grantham handed over divorce papers to his on-screen wife, Angie – broke all soap episode ratings, pulling in over 30 million viewers.
His other most memorable scene was shown in 1989, when he was apparently fatally shot with a gun concealed in a bunch of daffodils after meeting Michelle Fowler for a final rendezvous next to the canal.
A former soldier, Grantham learned to act while serving a 10-year prison sentence for the murder of a German taxi driver while on army service.
He won his EastEnders role seven years after serving his time, after playing a small role in a 1984 episode of Doctor Who.
Grantham finally left the show in 2005, coinciding with the soap’s 20th anniversary.
News Source SkyNews