The winner, who could be an individual or a syndicate, has a 180-day deadline to pick up the prize, meaning they have until 11pm on December 5, according to lottery rules.
If the ticket holder, with winning numbers 5, 11, 22, 34 and 40 and lucky stars 9 and 11, fails to come forward, the money and the interest generated will go into the National Lottery Good Causes fund.
A Camelot spokesman again urged people to "triple-check" their tickets.
Earlier this week Camelot urged those who have bought tickets in the relevant area to check 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," a Camelot spokesman said.