The footage (pictured) backs up a recent study that grey seals have a penchant for young porpoises with a thick layer of energy-packed fat. And that far from scavenging on dead animals, they attack healthy creatures
Just went you thought it was warm enough to go into the water...

Killer seals have been spotted off the British coast.

On four separate occasions, grey seals have been spied feasting on harbour porpoises that they have killed.

Video footage of one of the attacks shows a male contentedly ripping chunks of blubber off his prey, as the water all around turns bloody.

Although killer seals are known to lurk in the waters off the continent, this is the first time they have been seen around Britain.

And with porpoises similar in size to grown men, it raises concerns that swimmers could be next.

Tom Stringell, an ecologist for the Welsh environment agency, said that while people shouldn't panic, they should keep their distance from seals, on land and in the water.

The footage, the first in the world, was shot by a wildlife cruise company, off the coast of Pembrokeshire.

Dafydd Rees, who made the recording, said: 'I've been working on this stretch of the coast but have never seen anything like it before.

'It was really surprising to see.'

Grey seals are known for being playful and friendly. Found all around Britain's coastline, they can grow to 40 stone but normally feed on fish no bigger than salmon.

However, they have recently been blamed for attacks on porpoises on the continent.

Recent Dutch research concluded that hundreds of the dead porpoises bore the tell-tale marks of a seal attack.

The study said that the seals seemed to have a particular penchant for young porpoises with a thick layer of energy-packed fat.

And that far from scavenging on already dead animals, they were attacking healthy creatures.

The Utrecht University researchers said the seals may have developed a taste for porpoises after sampling some caught in fishing nets.

And with many of the mutilated carcasses washing up on beaches popular with swimmers, they warned people could be next.

They said: 'Keep on swimming and enjoying nature. However, people should be aware that the largest predator in our countries is the grey seal.

'These animals may reach 40 stone, are related to bears and have the teeth to go with that lineage.

'In the water, they are much more able than the most agile human swimmers and they have made the switch from eating fish to hunting porpoises, another mammal.

'To date, there have been no reports of serious attacks or wounds inflicted, but some people have been bitten by inquisitive or provoked seals.

'The advice would thus be to be aware that seals are not out there to cuddle and to keep some distance.'

Dr Stringell, who has done his own research for Natural Resources Wales, said that despite reports of seals eating porpoises off the coast of Belgium, France and the Netherlands, he was surprised to discover it happening here.

He said: 'We observed this happening on four separate occasions off the coast of Pembrokeshire.

'But it is unclear how long this has been going on for, and why.'

Natural Resources Wales said that growing seal numbers may mean there is increasing competition for food.

Alternatively, the attacks may be an example of 'opportunistic hunting' - with porpoises too close for hungry seals to ignore.

Dr Stringell, a senior marine mammal ecologist, said: 'Adult grey seals have also been known to attack, kill and eat juvenile grey seals, so this type of activity isn't totally surprising and is likely part of the natural cycle of life.

'Seals are unlikely to pose any direct danger to people. However, they are wild animals that possess sharp teeth and are a protected species, so they should not be approached on land or in the water, particularly during the autumn pupping season.'

This is not the first time the seal's cuddly image has been tarnished.

In November, it emerged that Antarctic fur seals had been raping king penguins. One penguin was then killed - and eaten.