A Euromillions ticket bought in the north east Derbyshire area and worth £382,155.70 is still unclaimed.
The owner of the winning ticket – who matched the five main numbers and one Lucky Star number in the draw on April 14 – has until Sunday to claim the prize.
The winning Euromillions numbers were 24, 32, 34, 35, 49 and the Lucky Star numbers were 1 and 2.
If nobody comes forward with the winning ticket before the deadline, the prize money, plus all the interest it has generated, will go to help National Lottery-funded projects across the UK.
A National Lottery spokesman 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 EuroMillions ticket in the north east Derbyshire 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.”
Anyone who believes they have the winning ticket should call the National Lottery Line on 0845 910 0000.