Shares of United Parcel Service plunged more than 15% Thursday after the company issued weak revenue guidance for the year and said it planned to cut deliveries for Amazon, its largest customer, by more than half.

United Parcel Service Inc. shares plunged after the company projected annual revenue well below expectations, telling investors that a long-awaited rebound in demand for its parcel services won’t arrive this year and prompting it to slash its low-margin business with Amazon.

Shares of United Parcel Service suffered their worst day ever on Thursday, after the package-delivery giant announced an agreement that would cut its business with Amazon.com Inc., its largest customer,