The FIA revealed on Friday night that Hamilton's car would be fitted with a new gearbox from Saturday's free practice session.
With this change coming before the mandatory six events are up, it means Hamilton will be moved down the order.
Teammate Valtteri Bottas is also having a new gearbox fitted, but as he already had a gearbox change penalty in Australia following his qualifying crash, he is allowed a free change for this weekend's race.
A Mercedes insider confirmed that Hamilton's gearbox had suffered a hydraulic leak during the race in Australia. It was only when it was inspected in the Bahrain paddock this week, under FIA supervision, that bearing damage was discovered.
The team has decided that it would have been too risky to use the gearbox again. Bottas's change is a routine one that every team is allowed to make after it has replaced a damaged gearbox at the previous event.
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