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,

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.