 The oldest and fiercest rivalry is with Millwall . The two sides are local rivals , having both formed originally around local companies , with players living in the same localities . The early history of both clubs are intertwined , with West Ham proving to be the more successful in a number of meetings between the two teams at the time , resulting in West Ham being promoted at the expense of Millwall . Millwall later declined to join the fledgling Football League while West Ham went on to the top division and an FA Cup final . Later in the 1920s , the rivalry was intensified during strike action which Isle of Dogs @-@ based companies ( i.e. Millwall fans ) refused to support , breeding ill will between the two camps , the bitterness of this betrayal enduring for years . In 1972 , a Millwall supporter died at New Cross station after falling out of a train during a fight with West Ham fans . 
