According to his agent, David Hahn, Mike - who played Frank Butcher in the BBC soap for just under two decades - suffered a suspected heart attack at his Spanish home.
Hahn told BBC News: "Mike was in fine fettle form. In fact only a couple of weeks ago we were having dinner over here.
"Mike, that particular day, went to see his consultant and had a full medical - and they gave him a clean bill of health."
He added: "He would see the funny side of any situation, no matter how black it was. Even now I'm sure he's laughing."
Describing it as "absolutely devastating", close friend and fellow comedian Frank Carson said: "We have just lost our friend Bernard Manning a fortnight ago. I just can't realise... this is devastating news."
He added: "This is horrendous news. Everyone in showbusiness will be shocked because this was one of the great friends of anything you wanted to do in showbusiness."
Controller of BBC Drama John Yorke said: "Mike's genius was to capture the heart of that flawed dreamer and make generations of viewers love him.
"Albert Square - and British television - will be a far poorer place without him, as will the lives of those who loved and worked with him throughout his extraordinary career."
Former Walford co-star Pam St Clement (Pat Evans) added: "It has come as a terrible shock - somebody larger than life as Mike was in person and character - he seemed indestructible."