The Baggies had agreed a £16m fee the Senegalese international, which would have been a club-record fee for the West Midlands side, smashing the one that brought Salomon Rondon to The Hawthorns last summer from Zenit St. Petersburg for a fee in the region of £12m.
Sources are reporting that West Brom pulled out of the deal after Sakho failed to pass his medical. Sakho had previously stalled on the move from London but it is not thought there is any correlation between the two issues.
Stringent medical failed by forward
After Sakho's injury plagued career for The Hammers, it was suggested that The Baggies' medical team had put in place a series of stringent medical tests to be undertaken by the forward before a move could be finalised.
It now seems that the 26 year-old has failed to pass those examinations.
Sakho has spent to last two season at Upton Park, scoring 15 leagues goals in 44 appearances. His 2015/16 campaign however was largely curtailed due to a recurring thigh injury.
Sakho developments again jeopardise Berahino
The twist in developments has once again thrown Saido Berahino's future into disarray. After a fee was agreed for Sakho was agreed on Sunday, it was thought that the England U21 international would be allowed to depart after a frustrating 18 months.
Now however, Berahino faces yet more upheaval with the prospect of taking part in West Brom's final pre-season games, before Tony Pulis' men travel to Selhurst Park on Saturday week for the opening day 2016/17 curtain-raiser against The Eagles.