SEATTLE — Amazon has reached a historic $2.5 billion settlement with the Federal Trade Commission, which said the online retail giant tricked customers into signing up for its Prime memberships and made it difficult for them to cancel after doing so.

The Seattle company will pay $1 billion in civil penalties — the largest fine in the agency’s history — and $1.5 billion will be paid back to consumers who were unintentionally enrolled in Prime, or were deterred from canceling their subscriptions, the agency said Thursday.

The surprise settlement comes just days after the trial began in U.S. District Court in Seattle this week. At the heart of the case is the Restore Online Shoppers’ Confidence Act, a 2010 law designed to ensure that people know what they’re being charged for online.

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