The prize was won in Doncaster in January – but no-one has claimed the cash – and if they don't come forward soon, they'll miss their chance to get hold of their winnings.
The ticket-holder won £1million in the EuroMillions draw on January 21 this year.
Their pay-out comes courtesy of the European Millionaire Maker raffle.
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But on July 20, the deadline to claim the prize will pass.
Anyone who remembers buying a ticket for the draw is urged to check in pockets and behind the back of the sofa.
Anyone who does win on the Lottery has 180 days to step forward.
Once the claim period is over, the ticket officially expires - meaning that even if a winning slip is then found, it's too late – meaning that one lucky punter could miss out on a whopping prize.
Any unclaimed prizes, plus any interest they might have accumulated in that time, are allocated to the National Lottery’s Good Causes fund instead.
The missing ticket was bought in Doncaster and the winning European Millionaire Maker code for this prize was TVSD 23735.
Camelot’s Andy Carter, Senior Winners’ Advisor at The National Lottery, said: “Time really is running out for the winner of this prize, but we are still hopeful that someone will come forward at the very last minute to claim the money.
"We’re urging everyone to check their old tickets one final time - the easiest way to do this is via the National Lottery app - or look anywhere a missing EuroMillions ticket could be hiding. This life-changing prize could really help to make dreams become a reality for someone out there.”
Andy continued: “Twenty-three of the 25 lucky UK ticket-holders have claimed their prizes from the special EuroMillions draw, and we’re hoping that these last two winners - one fom a ticket bought in Doncaster and the other in the Hinckley and Bosworth District (Leicestershire) - will now come forward so that they can start to enjoy their amazing win.