Ukraine secured gold in the women's quadruple sculls with a convincing win.
Kateryna Tarasenko, Nataliya Dovgodko, Anastasiia Kozhenkova and Yana Dementieva won by more than two seconds in a time of six minutes 35.93 seconds.
Germany took silver and the United States bronze, with Australia in fourth and 2008 champions China fifth.
"I was hoping that Great Britain's performance in the repechage, where they blasted through the last 500m, would enable them to perform with a bit more belief.
"Unfortunately, today was a showing on a par with the rest of their season."
The Great Britain team of Beth Rodford, Debbie Flood, Frances Houghton and Melanie Wilson never recovered from a poor start and finished a distant last.
Great Britain, with Flood and Houghton taking part in their third Olympic final, needed to make a fast start from lane one to have any chance of winning a medal, but were sixth and last at the 500m mark.
They continued to struggle and were never in contention to repeat the silver medals which Great Britain had won in the event in the previous three Games.
The result at Eton Dorney did not come as any real surprise, however, because GB failed to make the final at last year's World Championships.
After starting brightly, Ukraine had clear water at the 1500m stage and their smooth technique helped them to a 2.16 secs win over the Germans, with the United States a further 2.54 secs behind.
It was also Ukraine's first Olympic rowing gold medal.