According to Mary Nesbitt-Larking, who posted the snap on Facebook, when she ordered the meal she was informed the kitchen had run out of vegan cheese.
Calling the pizza a 'monstrosity', she posted a laughing emoji along with the caption: "They were out of vegan cheese and forgot my [replacement] pine nuts."
The photo made waves when posted on Facebook, with various commentators agreeing that it looked terrible.
In addition, a number of media outlets picked up on Nesbitt-Larking's snap, and shared her story. Some of the responses there were less supportive, including one which blamed the vegan for choosing to ditch animal products.
"She made her choice and sometimes there are consequences," it said.
New vegan menu
A spokesman for Pizza Express told Femail: "We take great pride in crafting delicious pizzas that our customers love - on this occasion, we let Mary down and apologize for her disappointing experience.
"We are following up to find out what went wrong and, by way of an apology, have invited Mary back to PizzaExpress with a friend, to try our new Autumn dishes and dedicated Vegan menu, which includes the new Vegan Puttanesca."