The Red Bull driver was forced to slam on the brakes to avoid making contact with the McLaren, and the matter was referred to the stewards as an alleged unsafe release.
Following the investigation, a €10,000 fine has been dealt to McLaren, with Piastri avoiding a grid drop.
In his defence to the stewards, Piastri noted that he had checked the pit-lane as best he could whilst leaving the garage, but that visibility was limited due to the angle of the cockpit and he did not see Verstappen until the championship leader appeared in his mirrors.
McLaren admitted the team had made an error allowing Piastri to come out in front of Verstappen, who said that it was a 'very optimistic' move.
The stewards concluded: "Having noted precedents for this type of breach, the Stewards are of the view that this breach warrants a more severe penalty than previously applied hence the penalty above is imposed."
Piastri will retain his P2 grid slot with Verstappen starting five places behind.