Could the mangled carcases of porpoises washed up on Norfolk beaches be further evidence that killer whales now prowl the seas off our coasts...?

Two weeks ago, bird watchers scanning the waters off Sheringham claim they saw one of the formidable mammals through their telescopes.

Experts said if genuine, the sighting would be the first time one of the 30ft predators had been seen off Norfolk.

But some naturalists have believed for years that the carcases of porpoises which sometimes wash up on our beaches have been killed by predators, rather than collisions with boat propellors.

One was found on Brancaster Beach a few days back, by environmental consultant Dr David Viner. The creature's head had been completely severed.

'It looks like it's been cut clean in half,' said Dr Viner. 'It also had strange markings on its tail that could be incisions in its flesh. They're not dogs, they're not foxes.'

Dr Viner said he hoped releasing the pictures would generate a debate over what had attacked the porpoise.

'I'm intrigued,' he said. 'Eighteen months ago, wthere were seals washed up with corkscrew cuts and there was a lot of talk about that.

'They found seals off the grand banks with similar marks and attributed them to Greenland sharks.'

Killer whales, or orcas, eat smaller marine mammals including seals and porpoises - both of which live off the North Norfolk coast and The Wash.

Norfolk naturalist and EDP columnist Percy Trett - currently recovering in hospital from an operation - has long believed killer whales were responsible for similar carcases found at Horsey and Winterton.

When news of the killer whale signting broke online, his son Dr Marcus Trett, a forensic ecologist based in Spain, said: 'So at last, we have independent sightings from North Norfolk that vindicate Percy Trett's diagnosis that the Winterton and Horsey porpoise deaths, based on the bite patterns, were down to orca.'

Today, Dr Trett said a killer whale would leave a distinctive sign.

'The thing I'd be looking at is the angle of the bite,' he said. It's got a narrow snout, so the bite is an acute triangle.

'If it's that sort of angle then you are potentially talking about orca - the killer whale.'

Dr Trett said the chances of killer whales being present of the Norfolk coastline could be as high as 70 or 80pc, as climate change altered the creatures' feding patterns.

'Normal patterns of feeding have been disturbed and they're exploiting new areas,' he said. 'They're intelligent animals, as patterns change they'll know where the shallows are they can run things towards.'