The actor, who also co-hosted the UK version of French gameshow Fort Boyard, appeared on EastEnders from 1985 to 1989 and returned to the BBC One series from 2003 to 2005. He played main character Watt, who was the landlord of the Queen Vic, the pub which serves as the focal point of the soap. During his initial four years, ratings reached up to 30M in 1986, when his character divorced his on-screen wife Angie.
A statement from his agent said he died on Friday morning after returning to the UK from Bulgaria to receive medical attention.
In addition to his role on the BBC One series, Grantham was a controversial character and was a constant figure in the British tabloids. In 1966, he was convicted of murdering a German taxi driver and in 2004 he was found to have performed webcam sex online from his EastEnders dressing room at Elstree Studios.
He is thought to have been in Bulgaria to star in a TV series called The English Neighbour.