The unclaimed ticket for the July 20 draw won the top prize in the game.
National Lottery players are being urged to check and double-check their tickets for the chance to claim this life-changing prize in time.
The winner matched all five numbers 23, 25, 33, 36, 37 and, the Thunderball, 10.
The lucky ticket-holder has until January 16 to make their claim.
A National Lottery spokesperson said: “We’re eager to find the mystery ticket-holder and unite them with their winnings – this amazing prize could really make a huge difference to somebody’s life. We’re urging everyone who bought a Thunderball ticket in this area to check their old tickets again or look anywhere a missing ticket could be hiding.”