If they still have their ticket but have not realised its worth, then they have until December 5 to claim the prize.
On June 8 two ticket holders shared the EuroMillions jackpot - each winning £63,837,543.60.
One of the tickets was bought in Belgium and the other in the Stevenage and Hitchin area of the UK.
Today, to try to prompt the lucky UK winner to come forward, lottery staff visited Hitchin bus station and Luton airport with some giant lottery balls.
Sixty local shops which sell lottery tickets will also get posters encouraging the ticket-holder to come forward.
A National Lottery spokeswoman said: "It's important that any player who thinks they may have misplaced their ticket and thinks they have a genuine claim to the prize notifies us in writing by 5pm on Sunday, July 8 at the latest."
According to lottery rules, the winner - who could be an individual or a syndicate - has a 180-day deadline to pick up the prize, meaning they must come forward before 11pm on December 5.
However, if they have lost their ticket then they only have 30 days from the date of the draw to do so.
Camelot can make a pay-out, at its discretion, where a ticket has been lost, stolen or destroyed - but only if there is sufficient evidence (and the claim is submitted within 30 days of the draw).
The winning numbers were 5, 11, 22, 34, 40 and the Lucky Star numbers were 9 and 11.
If members of the public are concerned about lost or unchecked tickets or if they have submitted a lost claim then they can call 0845 912 5000 or visit http://www.national-lottery.co.uk.