Russell held off his team-mate Lewis Hamilton to take a stunning win at Spa, but this breach of the regulations could cost him his victory.
The most likely outcome is that Russell is disqualified from the race entirely, handing Hamilton the win, Oscar Piastri second place and the final podium spot to the Ferrari of Charles Leclerc.
Why could George Russell be disqualified?
Speaking to media including GPFans after the investigation was announced, Mercedes chief Toto Wolff said: "It’s been reported to the stewards that there is a weight discrepancy. We will see what the stewards will decide."
Asked whether the one-stop strategy which won Russell the race on the track could have contributed to the weight issue, the Austrian added: "I can’t comment because the stewards haven’t given their verdict but clearly it was unexpected to run with one tyre throughout the race, so let’s see what they say."