Mercedes were forced to change the Briton's gearbox ahead of the event - and, as the previous unit had not completed six consecutive events, Mercedes and Hamilton will inevitably be penalised.
An official report from FIA Formula One Technical Delegate Jo Baeur read: "[Hamilton] did finish the last race in Baku and this gearbox change was before the six consecutive events expired.
"As this is not in compliance with article 23.5a of the 2017 Formula 1 sporting regulations, I am referring this matter to the stewards for their consideration.
"The team informed the technical delegate about the gearbox change on Tuesday, 04th July 2017, at 11:29 hours."
Hamilton, quickest in both sessions on Friday, has claimed pole for the past two years in Austria, and has also started from the front for three of the last four Grands Prix of 2017.
Despite that run he still trails Ferrari's Sebastian Vettel - with whom he clashed behind the safety car in Azerbaijan last race - by 14 points in the championship.