Britain lost their second straight match of London 2012, 73-65 to Canada, despite pushing their opponents close.
GB trailed Canada for much of the game but always stayed in touch, going in 36-32 behind at half-time.
The hosts then took a deserved lead in the fourth quarter but a series of errors allowed Canada to build a 73-65 lead with seconds remaining.
Jo Leedham and Natalie Stafford led GB's offence with 15 points, while Canada's Shona Thorburn netted 18.
Despite Britain finding success from the three-point line early on, they were heavily punished for giving away the ball too easily as they racked up 22 turnovers.
GB tied the scores at 61-61 with four minutes of the game remaining but lost their composure with the end in sight as Canada finished on a 16-4 run.
After losing to world number two Australia on Saturday, GB are under pressure to earn their first victory in the six-team group as they seek a top-four finish to advance from the group stages.
GB forward Kim Butler said: "This was the one to get. But we're going to bounce back. It's disheartening. This one hurts. But we need a positive outlook on this. We have a few more games in the tournament."
Head coach Tom Maher, who coached the Australian women's team to an Olympic silver medal in 2000, added: "I think the better team won. We had a chance but we weren't good enough to win so I think the result was just.
"We do not have A-grade reliable players yet."
Britain play reigning European champions Russia on Wednesday at 16:45 BST.