More than 46,500 people have sent an online letter to the coffee chain requesting it changes its add-on price for non-dairy alternatives.
Currently, Starbucks adds up to 80 cents to orders that swap cow's milk for soy, oat, almond, or coconut.
Animal-rights organization, PETA, said: "Starbucks should be encouraging customers to choose animal-friendly, vegan milks rather than cow's milk. Many other chains—including Tim Hortons, Stumptown Coffee Roasters, Noah's New York Bagels, Philz Coffee, and Costa Coffee—already offer dairy-free milk at no extra charge."
Back in March 2018, Rosalind Brewer, Starbucks COO, promised to increase the chain's plant-based options.
She stated: "We have a line that we'd love to share with our customers later this year, so it is in our plans. We know our customers ask us that repeatedly, and we'll be responding."
Since then, Starbucks UK has welcomed a range of vegan items to its menu, including a breakfast burrito, mac and cheese, and almond butter bar.