The 55-year-old Spanish manager had spent weeks deliberately deflecting questions about his future, insisting he had one year remaining on his contract.
As recently as Saturday he had told players he was staying. The leak, which emerged on Monday evening as Arsenal beat Burnley 1-0 to move five points clear, caught Guardiola entirely off guard.
A source told The Sun: "Pep is furious at the timing just before a game. He summoned the players to join a late night group call and confirmed he was leaving. He apologised for the way the news emerged. It took him by surprise. The players were told as recently as Saturday that he wasn't going so everyone is in a state of shock."
Gary Neville reacted with surprise on Sky Sports when the news broke, drawing comparisons to some of the most significant managerial departures in English football history.
"I'm surprised that it's broken right now. Not just because it's during Arsenal versus Burnley but it's the night before a Man City game. I think of being in school and the news of Kenny Dalglish leaving Liverpool, or Sir Alex Ferguson leaving Man Utd, and Jurgen Klopp leaving Liverpool recently. They are such huge moments in football, and I don't see it as news Man City would let get out or would want out."
Fabrizio Romano confirmed on Tuesday morning that Enzo Maresca, Guardiola's former assistant at the club, will succeed him. Manchester City can still win the title if they beat Bournemouth on Tuesday and Arsenal lose to Crystal Palace on Sunday. The final home game of the season against Aston Villa on Sunday will be Guardiola's farewell at the Etihad.
He leaves having won 20 major honours at the club across ten seasons, including six Premier League titles, the Champions League and this season's Carabao Cup and FA Cup.