The highest lottery prize ever to remain unclaimed - nearly £64m - will go to good causes if the winner does not come forward by 23:00 GMT.
The EuroMillions ticket for the 8 June draw was bought in the Stevenage and Hitchin area of Hertfordshire.
The missing ticket-holder matched all five numbers - 5, 11, 22, 34 and 40 - and the Lucky Star numbers - 9 and 11.
Shila Sachania, who runs a newsagents in Stevenage, said the £63.8m ticket is the talk of the town.
"People have been hunting high and low, searching in their cars and vans," she said.
"We had a customer, a lady customer you know, who she asked a friend or somebody, just to help her look in the house... and she said I'll give you some money for it."
The £63,837,543.60 jackpot is one of two winning tickets from the 8 June draw. The owner of the other ticket, from Belgium, has already come forward.
Anyone who believes they have the winning ticket for any of the draws within the 180-day deadline should call the National Lottery Line on 0845 910 0000.
Prizes that remain unclaimed after 180 days go to the National Lottery Good Causes scheme.