Kelly Slater denies close call with a shark: VIDEO

Surfing legend Kelly Slater has shot down speculation that he was captured on camera surfing next to a shark in Western Australia, saying he was "100 per cent sure" it was just the reflection of another surfer.

The 11-time world champion was shown on footage captured on his GoPro camera riding a barrel wave next to a mysterious silver shadow at The Box break in Margaret River.

When the footage was uploaded to the Association of Surfing Professionals' YouTube account on April 5, it sparked furious online debate about what the object was.

Shark photobombs surfer Kelly Slater?

Was it a great white shark? A dolphin? A reflection of Slater himself, or that of another surfer in the water?

Following his heat three win in the fourth round of the Margaret River Pro on Thursday morning, Slater said the rumours had "gone on long enough".

"It's clearly the guy paddling over the wave and, you know, when the face of the wave bends, it bends the light. It bends what you see in the image," Slater, 42, said.

He said he was "100 per cent sure" that the mystery object was "just a guy paddling".

"There were some dolphins out there around that time ... but they didn't ride any waves and we didn't see any sharks," Slater said.

Slater beat Michel Bourez and Fillipe Toledo in his fourth round heat to progress directly to the quarter finals in the event in Western Australia.

Kelly Slater.

Kelly Slater.