It was auctioned for charity in the United Arab Emirates, beating the previous record of £3.5m for a plate featuring the number 5.
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The successful bidder said he bought it because he "wants to be the best in the world".
Saeed Khouri, a member of a wealthy Abu Dhabi family, added: "I bought it because it's the best number."
However, he would not say which of his many cars would carry the record-breaking single-digit plate.
The oil-rich UAE began auctioning off so-called vanity licence plates last May.
Proceeds from the auctions go to a rehabilitation centre for victims of traffic accidents.
On Saturday, 90 licence plates were auctioned off, raising a total of £12m.
The previous five such events raised more than £25m.