If a £63.8m EuroMillions prize is not claimed by 6pm tonight, the area where the ticket was bought will be named.
Nearly a fortnight after the draw on June 8, the winner - who splits the jackpot of £128m with a person in Belgium - has yet to come forward.
The National Lottery said, if this continues, there will be a publicity drive in the region in a bid to find the winner, which could be an individual or a syndicate.
A National Lottery spokeswoman said: "This area is provided by Camelot's security team in line with its licence obligations and is designed to encourage tickets to be checked but also to enable the ticket-holder to remain anonymous if they wish."
It would be the longest period of time such a large prize had gone unclaimed if the 6pm deadline passes, she added.
The ticket-holder has 180 days from the draw to come forward, making 11pm on December 5 the final cut-off point. Afterwards, the £63,837,543.60 - and the interest it has generated - would go to the National Lottery Good Causes fund.
The prize pot was so large because it came as a result of a double rollover.
The spokeswoman added: "There are a variety of reasons people don't come forward - they might be on holiday, they might not play regularly, or sometimes in the case of syndicates, they only check their tickets once a month."
The haul puts the lucky winner on a par with other multimillion-pound jackpot winners including Colin and Chris Weir , from Largs in North Ayrshire, who won £161m last July.
Other big winners include a ticket-holder who banked £113,019,926 in October 2010 but decided not to go public, and Dave Dawes and wife Angela, from Wisbech, Cambridgeshire, who claimed £101,203,600 last October.
Camelot can make a pay-out, at its discretion, where a ticket has been lost, stolen or destroyed - but only if a claim is submitted within 30 days of the draw and if there is sufficient evidence.
