Adam Gemili's bid for another gold at the World Junior Athletics Championships ended in disappointment after Great Britain's sprint relay team were disqualified from Saturday night's final after a stumble by Emmanuel Stephens.
Stephens, who was running the opening leg, tripped over before he could hand the baton on as the GB youngsters' bid for glory in the 4x100m final in Barcelona ended prematurely.
Gemili, who clocked a personal best of 10.05 in winning Wednesday's 100m final and is part of the Team GB squad for this summer's Olympics, was due to run the anchor leg.
GB, who qualified second fastest from the semi-finals behind Japan in a time of 39.09, picked up the baton and carried on to finish the final in last place but were later disqualified by the judges.
The United States took the victory in 38.67seconds, with Jamaica second and Japan third.
There was also disappointment for the GB women's 4x100m relay team after they failed to finish with victory seemingly in their sights.
GB were leading at the final changeover but failed to exchange the baton cleanly and were left watching on as the US claimed another triumph.
Earlier in the evening Liverpool teenager Katarina Johnson-Thompson was unable to add to her medal haul in Barcelona after she failed to qualify for the final of the 100m hurdles.
Johnson-Thompson, who won long jump gold on Friday, could only finish fifth in her semi-final heat in a time of 13.94 seconds.