The 2003–04 season was the 89th season of the Isthmian League, which is an English football competition featuring semi-professional and amateur clubs from London, East and South East England. It was the last season for the Isthmian League as a feeder for the Conference Premier. At the end of the season, the Premier Division was replaced as a level 6 league (along with the Northern Premier League Premier Division and Southern Football League Premier Division) by the newly formed Conference North and Conference South. The Premier Division lost more than half its clubs to newly formed divisions, and the two regional divisions had a number of their clubs promoted to the Premier Division to replace them. Thus, the Isthmian League divisions downgraded to 7-9 levels.