Sunderland chairman Ellis Short fired Martin O'Neill over the phone on Saturday.
The Daily Mail says Short and O'Neill had been in conflict over the former Aston Villa manager's January buys and spoke on the telephone hours after the game on Saturday, when O'Neill was told his contract was being terminated.
He and his wife, Geraldine, then travelled to the club's training ground to clear his desk. The couple, who had moved into a house in Durham, are expected to return to their family base in the south.
The former Leicester and Celtic boss had been in charge for 15 months after he was appointed by Short as Steve Bruce's successor, weeks after Niall Quinn's departure as club chairman.