Players are being urged to check and double-check their tickets for the chance to claim the prize in time.
The owner of the winning ticket matched all five numbers and the Thunderball in the draw on Wednesday 2 April.
The numbers were 11, 18, 21, 29, 30 and, the Thunderball, 4.
The lucky ticket-holder has until 29 September to make their claim.
Anyone who believes they have the winning ticket should call the National Lottery Line on 0845 910 0000.
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