This is the terrifying moment a tourist swimming in the crystal clear waters of a rainforest river in Brazil swims straight into a huge anaconda.

Nail-biting footage of the encounter shows the giant snake lurking near the bottom of the Formosa River in the state of Mato Grosso do Sul.

But as the woman swims towards it she seems oblivious to the risk she is running until her friends on the river bank call out to her.

They can be heard shouting: “That’s right! It really is an anaconda! Come here!”

Moments later just as she is about to swim right over the snake it suddenly turns tail and swims away.

Realising the danger she is in, the swimmer does the same and rushes towards her pals as the footage ends.

The extraordinary clip was filmed by local lifeguard Davi Borge and has been seen more than three million times since it was shared online.

Green anacondas are among the most powerful predators in the world and can reach 30 feet (9 metres) in length and weigh 550 lbs (250 kilogrammes).

Although they are non-venomous they gradually suffocate their prey with their powerful coils and drag them underwater to drown.

But local guide and snake expert Vilmar Teixeira said the snake was probably more scared than the swimmer.

He said: “The anaconda ran away faster than the woman.”

Teixeira added: “Here in my region there is a great abundance of food. Seeing the size of the snake, what it eats are fish.

“The anaconda got scared and turned back, seeing the person as a large predator.”