Setback for Hamilton after strong practice sessions
FIA Sporting regulations state that gearboxes must last six races before a change is allowed but the unit Hamilton used in Baku had not completed the required six races.
If a driver does not finish a race, at their next event, they are allowed to take a change of gearbox, penalty free, but Hamilton does not qualify for this as he finished fifth in Baku.
Mercedes knew on Tuesday
In the FIA notice, it was announced that Mercedes had informed FIA Technical Delegate, Jo Bauer, of its intention to swap the unit on Tuesday morning.
It read: "The above driver, (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 One Sporting Regulations, I am referring this matter to the stewards for their consideration."
"Extensive damage" after Vettel collision
It is not clear when Hamilton's gearbox suffered the damage which has forced the change, but in the aftermath of the collision with Sebastian Vettel in Baku, team boss Toto Wolff said that the rear of Hamilton's W08 had received "extensive damage" when Vettel hit the rear of the Mercedes out of Turn 15.
At best, Hamilton will start the race in sixth position, which is where he won the 2014 British Grand Prix from.
As a result of the penalty, Vettel is now favourite to extend his championship lead in the Ferrari, which currently stands at 12 points.
In the 2014 race Hamilton could only qualify 10th but after a fast start was on the tail of the team-mate Nico Rosberg and went on to finish in second place.