The National Lottery is urging players of the EuroMillions Millionaire Raffle to check their tickets as the lucky £1m winner from the draw on Tuesday 25 February has yet to come forward.
UK players automatically receive a raffle code printed on their lottery ticket.
The money will go to good causes in August unless the winner is found.
A National Lottery spokesperson said: "We're desperate to find this mystery ticket-holder and unite them with their winnings.
"Try checking in the pockets of clothing, in wallets, bags and down the back of the sofa.
"We have the champagne on ice and our fingers crossed that the lucky winner comes forward to claim their win."
The person who believes they have the winning ticket - with the code ZXJ180326 - has a deadline of 180 days to claim the prize by calling the National Lottery on 0845 910 0000.
Can you imagine the difference that would make in someone’s life? Can you imagine what a difference it would make in your life?
The good news is if this amount is not claimed it won’t disappear into the organisation it will go to good causes.