If you win a EuroMillions jackpot the first thing you need to do is contact the National Lottery to let them know you have the winning ticket. They will guide you through the claim process and will arrange an appointment to validate the ticket. You have 180 days from the date of the winning draw to claim your prize.
If you bought a ticket from a retailer it's very important to keep it safe, as the National Lottery will need it to be able to validate the win. Things get a lot more complicated if you lose the winning ticket or it gets damaged to the point where it can't be verified.
If you played online you won't need to worry about this; you'll just need to log in to your player account, where a very welcome message about your win should be waiting for you. You’ll then just need to contact the National Lottery to begin the claim process.