A EuroMillions prize of £262,109.60 has yet to be claimed in Leicestershire.
The search is on to find the owner of the winning ticket, bought in the Hinckley and Bosworth borough, before the time to claim runs out.
The ticket buyer, who matched the five main numbers and one Lucky Star number in the EuroMillions draw on Friday, February 1, has five months to claim the money.
National Lottery players are being urged to check and double-check their tickets.
The winning EuroMillions numbers on that date were 5, 21, 34, 37, 38 and the Lucky Star numbers were 1 and 6. The lucky ticket-holder has until Wednesday, July 31, 2013, to make a claim.
If no-one comes forward with the winning ticket before the prize claim deadline the prize money, plus all the interest it has generated, will go to the Good Causes – adding to the £29 billion already raised through sales of lottery tickets since The National Lottery launched in November, 1994.
A National Lottery spokesperson said: "We are 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.
"We are urging everyone 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 the win."
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.
