The young Foxes could not follow up their 1-1 draw with Brighton & Hove Albion with a win, as two goals in six minutes helped the Hammers to victory.
After a tight first period, midfielder Sean Jordan Adarkwa broke the deadlock on 58 minutes, before Alfie Lewis, creator of the first goal, struck shortly after.
Forward Kairo Edwards-John replied on 69 minutes, but the Foxes were unable to find a leveller in the capital.
Currently sixth in the south table, the Under-18s next face Aston Villa in the U18 Premier League at Belvoir Drive on Saturday 24 February (11am kick-off).
The details…
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Stolarczyk, Gill, Elewa-Ikpakwu (Trialist 62), Thandi, Ughelumba, Heaven, Leshabela (c), Tee, Pennant, Shade, Edwards-John (Meite 82)
Subs not used: Davies, Fielding, McLeod
Goal: Edwards-John 69