Two giant monsters of the deep are waging a war two kilometres beneath the surface of the Antarctic Ocean, scientists have discovered.

The mysterious colossal squid and its foe the Antarctic toothfish have been found to feast on one another in a study published in the Journal of Natural History.

Colossal squid were first discovered in 1925, but very little is known about. They are believed to grow up to 14m in length and weigh up to 750kg – although most are probably much smaller. It is a slow-moving ambush predator, using its large eyes to detect prey rather than actively hunting.

The Antarctic toothfish, meanwhile, is shark-like in appearance and grow to around 2m in length. They are voracious predators that normally eat squid, fish and even their own offspring. It is thought they fill the ecological role sharks play in other oceans.

While colossal squid have been found to feed on toothfish in the past, scientists now believe this is a staple part of their diet. Not only this, the researchers from the Centre for Environment, Fisheries and Aquaculture Science in Suffolk found toothfish attack and eat colossal squid right back.

Study author Vladimir Laptikhovsky told New Scientist: "Literally nothing is known about the colossal squid. Taking into account the size of adult squid, the toothfish probably is its most common prey species, because no other deep-sea fish of similar size are available around the Antarctic."

The team examined more than 8,000 toothfish that had been caught by fishing vessels over three years. Seventy one specimens had been attacked by colossal squid – showing scratches made by the squid's suckers and deep gouge wounds from their beaks.

Researchers also found remains of a colossal squid inside the stomachs of 57 of the toothfish. The team believes toothfish pick on juvenile, wounded or old squid.

"Data collected onboard two South Korean longliners from 2011-14 targeting Antarctic toothfish provided insights into trophic interactions between two Antarctic top predators: the colossal squid Mesonychoteuthis hamiltoni and the Antarctic toothfish Dissostichus mawsoni," the authors wrote.

"Adults of each species opportunistically prey upon weakened representatives of the other species: squid will feed on longline-caught toothfish, and toothfish on dying and dead squid."