Scientists have revealed that there is not one but two types of killer whale living in UK waters.
Each differs in its appearance and diet, with males of one type being almost two metres longer than the other.
The killer whales could be at an early stage of becoming two separate species, the researchers say.
The international group of scientists has published its results in the journal Molecular Ecology.
"It's exciting to think about two very different types of killer whale in the waters around Britain," says Dr Andy Foote from the University of Aberdeen, UK, who undertook the study.
"Killer whales aren't really a species that we think of as being a regular visitor to Britain, but in fact we have two forms of these killer whales in our waters," he told the BBC.
Scientists have found different forms of killer whale that occupy particular niches in the Pacific and the Antarctic, but this is the first time that they have been described in the North Atlantic.
Dr Andy Foote undertook the study along with colleagues from universities and museums in Denmark and the UK.
Killer whales (Orcinus orca), otherwise called orcas, live in family groups called pods.
As the largest member of the dolphin family, killer whales are known for their intelligence and range of hunting behaviours.
There was very little prior to this study to suggest that different types of killer whale would be found in the North Atlantic.
However, Dr Foote and colleagues studied teeth from remains of killer whales stranded over the past 200 years and found a difference in tooth wear.
"We found that one form, which we call 'type 1' had severely worn teeth in all adult specimens," explains Dr Foote.
"The other form, 'type 2', had virtually no tooth wear even in the largest adults."
In the wild, killer whales that "suck up" herring and mackerel display this tooth wear.
Knowing this, the researchers suspected a difference in diet and ecological niche between the two groups.
Using stable isotope analysis that gives clues to the orcas' diet, the scientists found that type 1 is a generalist feeder, consuming fish and seals.
Type 2, on the other hand, is a specialist feeder that scientists suspect exclusively feeds on marine mammals such as small dolphins and whales.
This specialisation for alternate ecological niches has also resulted in a difference in shape and appearance.
"The two types also differed in length, with type 2 adult males being almost two metres larger than types 1 males," Dr Foote says.
The researchers also found that colour, pattern and number of teeth vary between the groups.
Dr Foote says the fish feeding type 1 killer whales are found across the North East Atlantic and around Britain.
The cetacean hunting type 2 killer whales are regularly seen off the west coast of Scotland and Ireland.
Genetic analysis indicates the two types belong to two different populations.
"Type 1 specimens were from closely related populations, but the type 2 whales were more closely related to a group of Antarctic killer whales," Dr Foote explains.
Comparing the findings with studies on killer whales around the world shows that killer whales have radiated to fill different ecological niches.
"It's similar to how Darwin's finches have adapted to different ecological roles in the Galapagos, but on a larger scale," Dr Foote notes.
He suggests this could be an important discovery for the future of the animals.
"They seem to have occupied completely different ecological niches and have started to diverge morphologically. This divergence may eventually lead to the two types becoming different species."
He also recommends the two types be considered "evolutionary significant units" and monitored separately in order to more effectively conserve one of the oceans most charismatic animals.