Great Britain's Adam Barrett and Thomas Laxton have failed to reach the 100m butterfly semi-finals by less than 0.1 of a second at the World Championships in Kazan.
Defending champion Chad le Clos, who is an Olympic champion in the 200m event, came second in the final heat clocking 51.83s.
Laszlo Cseh, who beat the South African to win gold in the 200m butterfly earlier this week, comfortably touched the wall first in 50.91s in heat six.
In the same heat, British duo Barrett and Laxton, who is making his Worlds debut, finished fifth and sixth respectively, which was not enough to book them a semi-final spot.
Cseh heads into tonight's swim as the fastest in the field, while Le Clos is eighth.
