Tourist Julie Roberts, 56, had just finished swimming off Levante Beach on the Costa Blanca when she set eyes on the fearsome creature.
As she left the water, Julie, from Cheltenham, Gloucestershire, told The Sun that lifeguards immediately started blowing whistles to warn other swimmers.
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She said: “We looked over and could see two fins coming towards the shoreline. There was a snorkeller just feet away from this animal and he didn’t realise. It was terrifying.”
Another holidaymaker shared his video of the shocking sighting on Twitter, commenting that he had “never seen so many people get out of the water so quick in my life”.
— MIcky (@shanksy03) September 14, 2017
Julie added: “There was another lady swimming on her own the other side and she was totally oblivious to what was going on.
“Everyone was shouting at her to try and get her attention and I have never seen anyone move quite so quick as she did when she realised.”
Julie – who estimates that the shark was around 8ft long – said that, while it appeared to be in some distress in the shallows, it was able to get itself back out to sea.