British surfer Tom Butler believes he's surfed the biggest ever "monster wave" in Nazaré, Portugal.

The claim will now have to be submitted to World Surf League in order to be officiated but, if the video is anything to go by, it seems Butler may have got himself a new record.

The surfer estimated the wave in Nazaré was about 30.8m, which breaks the record currently held by Brazilian Rodrigo Koxa, who surfed an 24m wave.

"It's the biggest wave surfed in the world this season and could beat the current big wave world record. When I was going down I thought, 'Holy f***, this is the fastest I've ever been'," Butler said, quoted by the Daily Mail.

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"Time kind of slows down, it's instinct really. I was doing everything to hold my balance. That's when you're training comes in - I do lots of gymnastics training to help me keep balanced."

Butler believes the wave was about 120m high.

"You don't really see it behind you because you're so concentrated on what's ahead of you.

"I reckon it was maybe 90 or even 100ft. I haven't had a ruler handy while in Portugal but when I get home I'll try to figure it out. It was a monster, I know that for sure."

Butler surfed the wave on Friday and says conditions were just right.

He won't find out whether he actually broke the record before April.