Cameron, who has previously stated he is an Aston Villa fan, said he supported West Ham during a speech in south London on Saturday. The Premier League teams play in similar colours.
Cameron blamed his mistake on a "brain fade" and said "these things sometimes happen when you are on the stump."
Cameron later told Sky News that "by the time you have made as many speeches as I have on this campaign, all sorts of funny things start popping out of your mouth."
Cameron has said he began supporting Villa after watching the team beat Bayern Munich in the European Cup final in 1982, at a time when his uncle — William Dugdale — was chairman of the English club.
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