Players are being urged to check the unique Lotto Millionaire Raffle code on any tickets from October 10, 2015.
The Lotto raffle creates a guaranteed two millionaires each week - but the drew on October 10 had 25 £1m winning codes.
The code for the winning ticket that was bought in Leeds is BLUE 2873 7030.
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 Lotto ticket in this area to check their old tickets again or look anywhere a missing ticket could be hiding.
“Try checking in the pockets of clothing, in wallets, bags and down the back of the sofa – someone out there could literally be sitting on a fortune. We have the champagne on ice and our fingers crossed that the lucky winner comes forward to claim their win.”
The lucky winner has until April 7, 2016 to claim their prize.
If nobody comes forward before the deadline, the money will go to help National Lottery-funded projects across the UK.