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,

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.

UPS projected a decline in revenue for the current year, with a key factor being plans to phase out more than half of the business it does with Amazon over the next 18 months.