A video posted on YouTube shows the incredible moment a lifeguard spots a drowning child in a packed wave pool and saves his life.

The video, which highlights the risks of leaving children unattended in swimming pools and is thought to have been filmed in the US, shows a young boy in the middle of the pool being thrown from his rubber ring by a wave.

The boy can be seen thrashing in the water as he struggles to stay afloat.

A lifeguard then blows her whistle and dives into the pool to save him.

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The man filming the footage can be heard saying to a member of pool staff: "That was real.. I mean that was way out.. have you stopped it yet?"

Water safety experts warn that almost half of children who drown do so within 20 metres of an adult.