A beach in Devon was evacuated after a shark fin was spotted by a member of the public around 20-30 metres offshore between the red and yellow flags.
Swimmers and surfers were forced to abandon Croyde beach after it was put on red alert when the fin was seen at around 5.30pm on Thursday.
RNLI lifeguards asked people to leave the water as a precaution as the fin was inside an area where it is safe to swim.
With heavy rain and poor visibility, the lifeguards could not confirm what the sighting was.
Surfing teacher Karma Worthington, who was on the beach at the time, told This is North Devon: 'I was teaching a surf lesson and I just saw something about 100 metres out. I thought it was a seal or dolphin, then I saw it again and it was pretty big and the lifeguards and a few people were saying it was a shark.'
Conservation charity The Shark Trust told the Mirror it was 'highly likely' that it was a basking shark.
An RNLI spokesman said: 'It's not unusual for there to be sightings of sharks or other large mammals off our coasts and while these species are harmless, RNLI lifeguards on the beach maintain a watchful eye for all aquatic hazards in the water.'
The Cornwall Wildlife Trust recently said basking sharks were back in Cornish waters in higher numbers than last year, with 28 spotted in June alone. The sharks are known for being 'gentle giants' and are generally harmless filter feeders.
Video: Man captures moment shark jumps out of water next to surfer in Florida
Deadly shark attacks fisherman's boat off Cornwall coast
Tourists stunned as shark washes up on British beach
Want to know which animals are the most harmful? Discover the world's deadliest creatures below...