Social media videos show black smoke billowing from a luxury yacht on fire in St Tropez in the French Riviera on Thursday.
The blaze took place on the Sea Lady II, a 41m boat which sleeps up to 10 guests and can be chartered from €65,000 (£56,000) a week.
Firefighters initially struggled to get on board to contain the fire, while emergency services quickly moved away surrounding yachts and put up anti-pollution barriers.
Officials said the boat was likely to sink in the port and the cause of the fire is still being investigated.