Prince William will open the Ring O'Fire race in Anglesey, Wales, on Friday (30.08.13).
The British monarch will start the 135-mile three-day marathon, which sees competitors circumnavigate the island of Anglesey, where the prince has been based while working as a search and rescue helicopter pilot.
Kensington Palace said in a statement: "His Royal Highness will meet a number of the runners and their families and a selection of volunteers involved in the administration of the race before officially starting the Ultra Marathon."
The Duke - who has been posted in Anglesey since 2009 - is currently joined on the island by the Duchess of Cambridge and their newborn son, Prince George.
William has recently thanked residents for their kindness and support since his family moved to the area.
He said: "I know that both of us will miss it terrible when my search and rescue tour of duty comes to an end next month.
"From the bottom of my heart, thank you for making my wife and me so welcome when we arrived here, as you do thousands of visitors each year."
The Duke and his family will next move to London, where he is expected to undertake full time royal duties.
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