Katy Sexton's attempt to defend her world 200m backstroke title came to a premature end as she failed to reach the final in Montreal.
Sexton's year has been disrupted by illness
Portsmouth-based Sexton finished 12th fastest in the semi-finals in a time of two minutes 13.41 seconds, 0.86secs outside the final qualifying spot.
It was nearly five seconds slower than her two-year-old British record.
"I am not 100% fit and I am not 100% healthy. For me to get into the semi-finals was great," said Sexton.
"I would have liked to go faster than I did in the heats. I am hoping it will go better at the Commonwealth Games trials next week.
"Nothing went wrong. I didn't expect to qualify because I have had a tough year. I have had overtraining syndrome."
Despite entering the Olympics in Athens last year as world champion, Sexton could finish only seventh in the final.
And her display in Montreal was the latest disappointment for the British team, who went into the World Championships with high hopes after winning eight medals two years ago.
So far they have only Caitlin McClatchey's 400m freestyle bronze, although Rebecca Cooke will hope to add to that after quailifying fourth fastest for Saturday's 800m freestyle final.
"I don't think you can place responsibility for a whole the team on the performance of a single athlete," said Cooke.
"I think everyone in the British team has given it their best shot - nobody has left anything in them. I am just here to do my races and concentrate on those just like everyone else is."