The 44-year-old former England defender has been in the job for three years and won the League One title in 2012.
Life in the second tier has been harder, though, and Athletic currently sit bottom of the table, some four points short of safety.
The club have yet to officially confirm Powell's departure two days after the FA Cup quarter-final loss at Sheffield United.
Charlton are due to face Huddersfield in the Championship on Wednesday night.
Source : PA
Source: PA