The £63.8 million Euromillions prize won by a UK ticket holder remains unclaimed nearly a fortnight after the draw.
There were two jackpot winners in the June 8 draw - one in Belgium and one in the UK. But the owner of the British lucky ticket - who could be an individual or a syndicate - is yet to come forward to collect the £63,837,543.60 prize.
If the prize is not claimed by 6pm on Thursday, the area in which the ticket was bought will be named by the National Lottery. There would then be a publicity drive in the region on Friday in a bid to find the winner.
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 money - and the interest it has generated - would go to the National Lottery Good Causes fund. The prize pot was so large because 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 £161 million 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 payout, 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.