Rare footage shows dozens of killer whales targeted two adult Gray whales in an 'never-before-seen' attack.

Video shows two dozen Bigg's orcas joining together for the relentless attack - which lasted five hours.

This was the first observed attack on adult gray whales in Monterey Bay in more than 30 years documenting encounters for the California Killer Whale Project.

Researchers at Monterey Bay Whale Watch said: “Usually killer whales will hunt gray whale calves as they head up to their northern feeding areas with their moms.

"But these were not calves - they were huge adult gray whales."

The agency spokesperson added: “We were able to observe the unique hunting strategies of the pods and the rarely seen defensive strategies of the two Grays.

“The battered gray whales eventually made it to shallow water, and the orcas broke off."

Bigg’s orcas, formerly known as West Coast Transient orcas, are a subtype described by researcher Michael Bigg.

They feed exclusively on marine mammals and tend to travel in smaller groups than so-called resident orcas.

The incident was captured on video in Monterey Bay, California, USA, on March 30.