A £58 million lottery fortune will be lost if the winning ticketholder doesn't come forward this week.
Camelot said a claim was made for the prize on a ticket bought in South Ayrshire, Scotland, following the draw on March 17.
But the money wasn't paid out, with Camelot refusing to elaborate on why the claim wasn't successful.
The huge sum of money up for grabs is still less than the highest jackpot to ever go unclaimed.
In 2012, a prize sum of £63.8m was lost when a ticket bought in the Stevenage and Hitchin area of Hertfordshire wasn't produced within the six-month deadline.
It comes as the jackpot for Saturday's draw is now a guaranteed £20m, which may trickle down to increase the size of smaller prizes if the jackpot is not won again.
Camelot's Andy Carter, senior winners' adviser at The National Lottery told Sky News: "The next Lotto draw will see an amazing £20m special 'Must Be Won' jackpot up for grabs.
"A 'Must Be Won' draw means there could be lots more winners winning lots more if the jackpot rolls down."
Speaking last month, he said: "We're desperate to find this mystery holder and unite them with this massive prize.
"We're urging everyone who might have bought a ticket in this area to check."
Last month, Camelot revealed details of a series of special draws for September where hundreds of millions of pounds will be up for grabs.
The move is a way to try and give retailers a helping hand as they struggle to get back on their feet due to the coronavirus pandemic.
Around a million people win a fiver in certain "Must Be Won" draws.
From Saturday, November 7, players who match two main numbers in a Lotto "Must Be Won" draw, where no one matches all six main numbers to win the jackpot outright, will also scoop a share of the jackpot, winning £5 in addition to their free Lucky Dip.
Saturday night's National Lottery Euromillions winning numbers were 06, 29, 43, 40, 22 and the Lucky Stars were: 07, 04.
The Millionaire Maker Selection winner was VQJJ48440.
In addition, the National Lottery Thunderball winning numbers were 02, 07, 34, 38, 39 and the Thunderball was 14.