The unclaimed £63.8 million EuroMillions prize has been linked to a ticket bought in Hertfordshire.
Its holder has still to come forward to accept the windfall, nearly a fortnight after the draw.
There were two jackpot winners 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.
A Camelot spokesman said: "National Lottery players are being urged to check, double-check and triple-check their tickets for the chance to claim this staggering prize."
This is now the longest time such a large win has gone unclaimed.
According to lottery rules, the ticket-holder - 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.
The 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 winner 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.
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.