Lottery operator Camelot is urging ticket holders to double check their tickets to see if they are a winner.
The ticket, bought Co Armagh, matched the five main numbers and one Lucky Star number in the EuroMillions draw on New Year's Day, Tuesday 1 January.
The winning numbers on that date were 2, 7, 8, 36, 49 and the Lucky Star numbers were 1 and 11.
The lucky ticket holder has until Sunday 30 June 2013 to make their claim.
"We're desperate to find this mystery ticket holder and unite them with their winnings - this amazing prize could really make a huge difference to somebody's life," a National Lottery spokesperson said.
"We're urging everyone to check their old tickets again or look anywhere a missing ticket could be hiding - this prize would certainly be a great start to 2013.
"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."
If no-one comes forward with the winning ticket before the prize claim deadline, then the prize money, plus all the interest it has generated, will go to the Good Causes - adding to the £29bn already raised through sales of lottery tickets since The National Lottery launched in November 1994.
Anyone who believes they have the winning ticket for any of the National Lottery draws within the 180-day deadline should call the National Lottery Line on 0845 910 0000.