One lucky punter matched the five main numbers in the Set For Life draw on June 8.
The winning Set For Life numbers on that date were 6, 13, 17, 37 and 44 and the Life Ball was 3.
Lotto bosses revealed the winning ticket was bought in the Scottish city of Dundee.
The matching ticket-holder has until December 5 this year to claim their £10,000-a-month prize.
Anyone not in possession of their ticket, for whatever reason, but who believes they have a genuine claim can still make a claim in writing to Camelot – but it must be within 30 days of the draw.
Senior winners’ advisor Andy Carter said: “We’re desperate to find this mystery ticket-holder and unite them with their winnings.
“This amazing prize could really make a massive difference to somebody’s life.
“The next 12 months could see the lucky winner banking £10,000 in every single one of those months.
“We’re urging everyone who bought a ticket in this area to check their old Set for Life tickets again – the easiest way to do this is via the National Lottery app – or look anywhere a missing ticket could be hiding.”
He suggested “checking in the pockets of clothing, in wallets, bags and down the back of the sofa”.
Mr Carter added: “We have the champagne on ice and our fingers crossed that the lucky winner comes forward to claim their win.”
It follows a similar appeal after another Set For Life prize-winning ticket remained unclaimed, having been bought in the South Holland district of Lincolnshire.
And a Lotto-winning ticket worth £1m was unclaimed after being bought in Co Antrim, Northern Ireland.
The latest EuroMillions draw – offering a jackpot prize of £14m – was made last night.
The main National Lottery draw comes tonight at 7.45pm, after two lucky Brits scooped £6.3m apiece in last Wednesday’s.
Even those sums were dwarfed, however, by the £55m EuroMillions prize won by an even luckier Brit the previous day.