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. Scroll down for video . 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 . 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. The footage (pictured), the first in the world, was shot by a wildlife cruise company, off the coast of Wales . Ferocious: Scientists said the seals may have developed a taste for porpoises after sampling some caught in fishing nets (stock image shown) and warned that people could be next . ‘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. The seals, which are normally prey for great white sharks, have been seen attacking and killing medium-sized blue sharks (pictured) While most people will be familiar with images of seals being violently smashed out of the water snatched in the jaws of a shark, it appears some are exacting their revenge on the ocean predators. Scientists have reported multiple cases of cape fur seals attacking blue sharks off the coast of South Africa. The seals, which are normally prey for great white sharks, have been seen attacking and killing medium-sized blue sharks, devouring their guts before leaving the rest of the carcass. Divers also report seeing the fur seals attacking other species of shark and rays. The behaviour has left marine biologists baffled as to why the seals, which normally hunt smaller fish, have turned to attacking the torpedo-shaped sharks. The blue sharks are around the same size as the seals and it is thought to be the first time a seal has been seen preying on such a large predator. It was first observed by marine photographer Chris Fallows, who runs a dive boat operation from Cape Town in South Africa. 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. Although killer seals are known to lurk in the waters off the continent, this is the first time they have been seen around Britain (Pembrokeshire marked) 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. Not such a gentle giant: Grey seals (stock image pictured) were 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 . Bloodthirsty: Experts have warned that seals are 'not out there to cuddle and advise to keep some distance . 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. Britain is home to up to 170,000 grey seals – almost half the world’s population. At up to 8ft 6in long and weighing 40st, they are the biggest and most common of the two types of seal found in the UK and can live for up to 35 years. Their thick blubber and waterproof fur insulate them from the cold, while high levels of haemoglobin – the part of red blood cells that carries oxygen around the body – allow them to stay under water for up to 16 minutes at a time. The largest of Britain’s carnivores, they have highly sensitive whiskers and acute sense of hearing to help them locate food – normally fish – with even blind seals having no trouble catching prey. Once hunted for their skins, blubber and meat, seals were the first animals to be protected by modern wildlife laws. They can be shot under licence by fishermen if they are damaging nets.
On four occasions, grey seals were spied eating porpoises they had killed . Although killer seas are known to lurk in the waters off the continent, this is the first time they have been seen around Britain . The footage, the first in the world, was shot by a wildlife cruise company, off the coast of Pembrokeshire .