The National Lottery windfall has been won with a ticket bought somewhere in Denbighshire for the 1 April draw.
It matched the Lotto Millionaire Raffle draw - AQUA 6323 7030 - but no-one has come forward.
The mystery ticket holder has until the end of September to submit their claim - otherwise the money will go to lottery good causes.
"We're urging everyone who bought a Lotto ticket in this area to check their old tickets again or look anywhere a missing ticket could be hiding," said Andy Carter from the National Lottery.
"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."

Vonn finished 0.70 seconds clear of Switzerland's Lara Gut, with Austrian Cornelia Huetter a further 0.55 seconds back in Altenmarkt Zauchensee.
The American also won Saturday's downhill event.
That equalled Annemarie Moser-Proll's record tally of 36 World Cup downhill wins.
The 31-year-old now has 73 overall World Cup wins - 13 short of Ingemar Stenmark's record.
Vonn leads the World Cup super-G standings by 80 points over Hutter, and is now within 38 points of Lara Gut in the overall standings.
"It's a perfect start to the new year," she said. "Hopefully I can just continue on in this direction and keep winning."
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The Local Government Association said authorities were reviewing fire risk assessments and the construction of buildings.
Staff are also working closely with tenants to review and offer fire safety advice.
Police say at least 30 people died as a result of the west London blaze.
Live: Latest updates on the Grenfell Tower fire
Lord Porter, LGA chairman, said: "Following the horrific fire at Grenfell Tower, councils with tower blocks in their local area have been working with their local fire service, and undertaking urgent reviews of their high-rise buildings.
"Fire risk assessments and the construction of buildings are being reviewed and double checks are being made to ensure remedial work recommended under previous assessments have been carried out."
Extra fire safety checks were immediately organised across local authorities, including Camden, Newham, Croydon and Redbridge.
In Leeds, which has 116 blocks, the council is carrying out a review of fire safety in all blocks as a matter of urgency, but reassuring residents  adequate checks are already in place.
In the Midlands, councils in Coventry and Birmingham, which have 38 high-rise blocks, say they have double-checked records and no buildings have cladding like the type used in Grenfell Tower.
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In Wolverhampton, the city council said that of the 36 tower blocks across the city, a number have external cladding.
The authority said it was "confident" the cladding is of a correct standard, but will undergo urgent checks with manufacturers.
Fire crews in Solihull have visited high-rise blocks to reassure residents.
Senior councillors in Liverpool will hold a meeting with social landlords on Saturday, while the fire service is reviewing its inspection processes for high-rise buildings and prioritising inspections of high-rise premises across Merseyside.
St Katherine's Court in Northampton, which was recently refurbished, was inspected earlier by the fire service.
Management said cladding used is a different type to that used on Grenfell Tower.
Meanwhile, fire services in Lincolnshire, Cambridgeshire and Humberside have also confirmed they will be visiting high-rise buildings.
And in Cornwall, which has one tower block, a meeting with residents and council and fire officers is planned for next week.
Lord Porter added: "Councils take the safety of residents extremely seriously. We need to know with certainty how this fire started and why it spread so quickly so that councils can start to take any action needed."
The search is on for the winner of an unclaimed winning lottery ticket worth £1m.
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Lindsey Vonn claimed her second win in two days at the Alpine Skiing World Cup event in Austria with victory in the super-G event.
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Councils across England are carrying out urgent reviews of high-rise buildings in the aftermath of the Grenfell Tower fire.