The Hammers' bid for Bacca could be in danger.
The AC Milan striker is currently weighing up a move to the Olympic Stadium, according to the son of Hammers co-chairman David Sullivan.
However, the report claims Barcelona are also keen to add to their attacking ranks and could lure Bacca back to Spain with the incentive of Champions League football.
Barcelona's Camp Nou home
The Catalan giants are said to see the former Sevilla star as a possible alternative to Atletico Madrid forward Luciano Vietto.
According to the Mail, Barca's move for the Argentine is proving troublesome and Sevilla, incidentally, have also been credited with an interest.
Juventus striker Mario Mandzukic was linked with a move to the Hammers on Monday (Mail) while reports in Spain have named Deportivo's Lucas Perez as a target, though Barca are again said to be keen.