West Ham fielded a strong team, including David James, Joe Cole and Jermain Defoe last night but struggled against a well-organised Wycombe side, managed by Lawrie Sanchez.
The Hammers fell behind after 19 minutes when a mistake from James allowed Gavin Holligan to tap the ball into the empty net. Within two minutes Wycombe had doubled the score, with a header from defender Roger Johnson.
Frederic Kanoute pulled one back 10 minutes before half-time and in the second half West Ham had several chances.
But Wycombe substitute Lewis Cook sealed the win for the Division Two side 10 minutes from time.
Despite the poor result, Robert Lee, who is on trial with West Ham, insisted the game was an important lesson.
He said: "It was like a typical Division One match. We have to match teams physically and nick results."
The midfielder is desperate to win a contract with the Upton Park club and added: "I have one big ambition left and that's to end my career at West Ham. I haven't signed yet but I'm hopeful. I want to help them back into the Premiership."