Not even the boost of an early goal could lift the Hammers, who crashed to their eighth consecutive defeat as Chesterfield claimed another prize scalp in the competition.
Goals from Colin Larkin and Caleb Folan gave them a thoroughly deserved victory and left West Ham to contemplate the mounting crisis at Upton Park.
It had all looked so different when Marlon Harewood volleyed his team into a fourth minute lead which was West Ham's first goal in 671 minutes of football.
Harewood connected sweetly with his right-foot volley from the edge of the area, but West Ham were soon pushed back as Chesterfield responded with spirit and belief.
West Ham looked like a side bereft of confidence as their midfield were second to the ball which put increasing pressure on a defence in which Anton Ferdinand made vital interceptions.
Keeper Robert Green almost spilled a low shot from Aaron Downes over the line and the visitors were relieved to hear the half-time whistle but there was no respite in the second half.
An equaliser looked inevitable and it came in the 54th minute when Folan drove a shot from 20 yards against the post and Larkin steered the rebound past the stranded Green to give his team the goal they richly deserved.
West Ham were defending desperately and Green made another excellent save to deny Kevan Hurst before Bobby Zamora volleyed just over in a rare attack by the Premiership outfit.
But against the run of play, West Ham should have regained the lead six minutes from time when Teddy Sheringham put Zamora through but he shot straight at Barry Roche.
The importance of that save was underlined three minutes later when a Chesterfield free-kick from the right was headed out but the ball ricocheted off Sherringham and into the path of Folan who smashed past Green from eight yards.
West Ham piled forward during three minutes of stoppage time but Chesterfield deservedly held on to claim another famous Cup victory and leave Pardew with mounting problems.