The BBC announced this afternoon that John, who played Jim Branning in the soap, has sadly passed away after suffering from a debilitating stroke seven years ago.
Tributes have been flooding in for the much-loved actor. Actress June Brown, who played his screen wife Dot, said: "I am so sorry that John has passed away but hope that he is now at peace after seven difficult years."
June continued: I shall miss him very much as I loved him dearly. My thoughts are with his devoted wife, Enda."
Natalie Cassidy, who played Jim's granddaughter Sonia, added: "I never had a real granddad so John was like an actual granddad to me. He was a lovely man and I learnt a lot from him."
Other kind words came from Dean Gaffney, who played Jim's grandson Robbie. Dean wrote on Twitter: "RIP John Bardon, forever in our hearts. Was privileged to not only work with you but to call you 'grandad'.... X."
Perry Fenwick, who plays Billy Mitchell, said: "I'm absolutely gutted. John was a lovely, lovely man to work with. I'll miss him".
Speaking on behalf of the programme, an EastEnders spokesperson said the cast and crew are "absolutely heartbroken" to learn of his death.
John starred in EastEnders from 1996 to 2011. His character Jim fell in love with Dot Cotton and they married in 2002. In 2007, Jim suffered a stroke - the storyline was created to account for John's own off-screen illness. When John was well enough to return, he featured in the show until 2011.
Tonight's episode of EastEnders will be dedicated to John.