Four-time Olympic medallist Siobhan Haughey pulled out of the World Aquatic Championships in Singapore with a back injury on Monday, leaving the Hong Kong swimmer unable to defend her women's 200m freestyle title.

The 27-year-old Haughey won freestyle silver in the 100m and 200m at the Tokyo Games in 2021 and took bronze in the same two events in Paris last year.

"I have pulled out of the world championships in Singapore due to an ongoing back injury," she said in a social media post on Monday.

"I'm sad to miss out on the opportunity to compete against the best in the world, but right now I have to do what's best for me," said Haughey, who took a four-month break from the sport at the beginning of this year and has only competed once since.

"As elite athletes, we're trained to push through the pain and fatigue to keep going," she wrote.

"Yet at certain moments, the brave and hard thing to do is to take care of your body and wait patiently. Luckily I have an incredible team supporting me."

Haughey also won the 200m freestyle at the Doha world championships last year.

She had been entered in the 100m and 200m free at the World Aquatic Championships in Singapore, where the pool swimming events will take place from July 27 to August 3. (AFP)