The EuroMillions ticket was purchased for the June 8 draw in the Stevenage and Hitchin area of Hertfordshire, and won a prize of nearly £64 million.
The winnings will go to charity if the ticket remains unclaimed by 11pm tonight (December 5).
The ticket matched all five main numbers - 5, 11, 22, 34 and 40 - and the Lucky Star numbers - 9 and 11.
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Newsagent's owner Shila Sachania told BBC News: "People have been hunting high and low, searching in their cars and vans.
"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."
A second winner from Belgium has already claimed their prize for the same draw on June 8.
Lottery prizes remain open for 180 days, before they go to the National Lottery Good Causes scheme.