City's youngsters headed to Little Heath Sports Centre in the capital buoyed by successive victories over Brighton & Hove Albion and Reading, but that run was halted on Saturday. Gideon Kodua put the Hammers ahead on 23 minutes, before George Earthy hit a second for the home side.
It was 3-0 to West Ham before the break, too, as Sonny Perkins was laid up by Kodua. The young Foxes, though, delivered an improved performance in the second half and got a reward for their efforts nine minutes after the interval as Chris Popov narrowed the arrears.
Despite their best efforts, Leicester's Under-18s were unable to mount a comeback at Little Heath and suffered only their second loss of the 2021/22 season. As a result, City drop to eighth in the early U18 Premier League South standings, on six points from their opening four matches.
Next Saturday, they welcome Newcastle United Under-18s to LCFC Training Ground in Seagrave for an 1pm BST kick-off.
The details
City: Doherty; Grist, Cook (Wilson-Brown 45'), Read, Cartwright; K. Pennant, Dawkins, Jesper Kutshienza (Javid 76'), Cover; Popov, Butterfill (Lewis 81').
Subs not used: Weeks, Jude Kutshienza.
Goal: Popov 54'.