Following final practice in Singapore, Hamilton was reported to the stewards for “an alleged breach of Appendix L, Chapter III of the FIA International Sporting Code.” 

Chapter III of the Sporting Code includes infractions relating to helmets, flame-resistant clothing, frontal head restraint (FHR), safety belts and the wearing of jewellery. 

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Hamilton was forced to remove his nose stud earlier this season after a long running stand-off with the FIA amid a clampdown on the wearing of jewellery while driving. 

The seven-time world champion finished 12th-fastest in FP3 as Mercedes struggled with tyre warm-up in the damp conditions.

The investigation comes on a weekend F1 is facing a brewing storm over the 2021 budget cap, after reports emerged that two teams may have broken last year's spending limit. 

Red Bull boss Christian Horner hit back at suggestions his team had voilated the 2021 financial regulations following statements made by rivals Mercedes and Ferrari. 

Horner warned that Red Bull would consider taking legal action if the "hugely defamatory" and "fictictious" claims were not withdrawn.