Washington DC - Billionaire Elon Musk's America PAC has been warned by the Justice Department that its $1 million daily giveaways to registered voters may violate federal law, various media outlets reported Wednesday.

Musk, the world's richest man, announced the contest on Saturday in Pennsylvania, one of the seven "swing states" that will likely determine who will become the next US president – Donald Trump or Kamala Harris.

Musk, a Trump supporter, announced plans to give $1 million to one registered voter in a swing state every day until the US election on November 5.

CNN and 24sight News said the letter from the Justice Department to Musk's political action committee warns that the $1 million sweepstakes may violate federal law, which prohibits paying people to register to vote.

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The Justice Department declined to comment.

The winners of the contest are chosen at random but they must be registered voters. They are also required to sign a petition that supports "free speech and the right to bear arms."

Danielle Lang, a professor at Georgetown Law School who specializes in election law, said earlier this week that the contest could be "subject to civil or criminal enforcement by the Department of Justice."

"It is illegal to give out money on the condition that recipients register as voters," Lang said.

"As the terms of this 'contest' to win $1 million require the recipient to be a registered voter in one of seven swing states (or to register if they have not already), the offer violates federal law."

Tesla and SpaceX CEO Musk has pulled out all the stops in recent weeks to support former president Trump, including donating $75 million to a pro-Trump political action committee and appearing at campaign rallies.