Sunderland have confirmed they have "parted company" with manager Martin O'Neill after Saturday's 1-0 defeat to Manchester United, left the club 16th in the Premier League table.
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A statement on the Wearsiders' official website said: "Sunderland AFC has announced that it has parted company with manager Martin O'Neill this evening.
"The club would like to place on record its thanks to Martin and wishes him well for the future. An announcement will be made in the coming days regarding a successor."
O'Neill, 61, took the reins at the Stadium of Light in December 2011 and guided the Wearsiders out of the bottom three and to a 13th-placed finish in the Premier League last season.
However, with Sunderland winless in eight games and currently just one point above the relegation zone, the club have acted to try and improve their fortunes in what has the makings of one of the most fiercely contested relegation battles in years.
The Wearsiders face tricky visits to European champions Chelsea and North East rivals Newcastle United in their next two games.
Former England boss Steve McClaren has emerged as the early bookies' favourite to replace O'Neill, with Paolo Di Canio, Roberto Di Matteo and Mark Hughes also among the rumoured contenders.