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Charlton were relegated to League 1 on Tuesday night.
Their fate was sealed by a goalless draw away to basement side Bolton, which left the Addicks 11 points from safety with three games left.
Charlton have been rooted in the bottom-three since December and they will now return to the third tier of English football after four seasons in the Championship.
Chief executive Katrien Meire said: “Everyone at Charlton Athletic Football Club is deeply disappointed that we have been relegated from the Championship.
"This is not the time for excuses. We apologise for our mistakes and now need to learn from them so that we can put firm foundations in place for the future that ensure the long-term stability of the club.
“We all acknowledge that this is a difficult time for the club but I and the club’s senior management team will start the rebuilding work immediately. Our sole aim is to be fighting for promotion next season.”
Tuesday saw the Addicks fail to score for the third time in four games.
Head coach Jose Riga said: “There was no solution on the pitch, no fluidity, no options and the accuracy was not there."
Charlton lost 1-0 at home to Derby last weekend when a 60th-minute Johnny Russell goal earned promotion-chasing County all three points. The Addicks host Brighton this Saturday.