The unclaimed £63.8 million EuroMillions prize has been narrowed down to a ticket bought in Hertfordshire.
Its holder has still to come forward to accept the windfall, nearly a fortnight after the draw.
This is now the longest time such a large win has gone unclaimed.
Two ticket holders hit the jackpot on June 8 - one in Belgium and one in the UK.
The owner of the British ticket, bought in the Stevenage and Hitchin area, will claim £63,837,543.60.
According to lottery rules, the winner - who could be an individual or a syndicate - has a 180-day deadline to pick up the prize, meaning they must come forward before 11pm on December 5.
A Camelot spokesman said: "We're desperate to find this mystery ticket holder and unite them with their winnings and we're urging everyone to 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."
If the ticket holder fails to come forward, the money - and the interest it has generated - would go to the National Lottery Good Causes fund.
The prize pot came as a result of a double rollover.
Other multimillion-pound jackpot winners include Colin and Chris Weir, from Largs in North Ayrshire, who won £161 million last July.
A ticket holder banked £113,019,926 in October 2010 but decided not to go public, and Dave Dawes and wife Angela, from Wisbech, Cambridgeshire, 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.
