Princess Beatrice will run and hike more than 30km to the summit of Mount Etna in Sicily today to raise money for a charity she co-founded.
The 28-year-old royal, who completed a 3,300m swim and 110km mountain bike this week as part of the Virgin Strive Challenge triathlon, will be joined on the active volcano by Sir Richard Branson and his children Sam, 31, and Holly, 34.
Beatrice set up the charity Big Change with the Branson children, who organised the Strive Challenge to raise money for the foundation.
She joined the Bransons for the final triathlon stage of the challenge on Monday and is expected to reach the summit of Etna alongside them later on Thursday.
The Virgin Strive Challenge began at the base of the Matterhorn in Switzerland 30 days ago.
Since then, a team of core participants - including the three Bransons - have hiked through the Alps, cycled the length of Italy and swam across the treacherous Straits of Messina from the mainland to Sicily.
The team, who were joined by around 60 other participants at the different stages, hope to raise £1.5 million for Big Change.
Big Change acts as a catalyst for a number of projects - including How to Thrive and Voice 21 - to accelerate their success and give young people the "necessary tools they need to thrive in life".
Sir Richard is also set to complete the entire Strive Challenge despite narrowly escaping a near-deadly bike crash less than two weeks before he set off at the beginning of September.
Speaking at the start of the challenge earlier this month, the 66-year-old said he was "very, very lucky" to escape the accident with cuts, a cracked cheek and a damaged knee.
But he said he was determined to finish the 2000km cycle, hike and swim.
The team are expected to reach the summit of Mount Etna late on Thursday afternoon.