A Norfolk mammal expert said today he was 'thrilled and excited' at reputable reports of the first-ever killer whale sighting off the county's coastline.

The animal was spotted breaching the water off Sheringham on Thursday morning, February 9, by two local bird watchers, including long-serving former Norfolk bird recorder Giles Dunmore.

Mr Dunmore and fellow birder Phil Vines were scanning the sea with telescopes at about 8.15am, looking for unusual ducks and geese heading for Norfolk from the frozen continent.

Mr Vines called out that he had spotted a harbour porpoise but when Mr Dunmore trained his telescope on the area to see it, he picked up something much further away.

'I can't estimate how far out it was but I suddenly saw an area of disturbed water and then this black hump came out of the sea and a great big, black dorsal fin - then it disappeared. It was heading eastwards,' said Mr Dunmore, 68, from Beeston, near Sheringham.

'I yelled out and Phil was desperately trying to see it. A few seconds later he yelled out too, with a fine Anglo-Saxon expletive.'

Mr Dunmore stayed at the site until driven indoors by the cold at about 10am, but neither man saw the whale again.

Mr Dunmore, a retired chief planning officer with Broadland District Council, has seen killer whales off Tenerife and Alaska. He said the mammal's dorsal fin 'like a backwards-sloping triangle', was particularly distinctive. He believes the Norfolk animal was a female.

Male killer whales, or orcas, can grow up to 9.5m long (31ft) and females 5.7m (nearly 19ft).

Mr Dunmore contacted county mammal recorder Dave Leech to discuss the sighting and will now be submitting a detailed written report and field sketch of the incident so that it can be officially recorded.

Dr Leech, who is based at the British Trust for Ornithology in Thetford, said a check of records stretching back to the beginning of the 1900s showed that there had never before been a killer whale sighting off Norfolk.

'Giles is an experienced person who has seen them before in other countries and as far as we are concerned, it's a reputable record,' he said.

'I was very excited, thrilled and shocked when he rang - and I wished I was at the north Norfolk coast right there and then.

'Within UK waters, the majority of killer whale sightings are on the Atlantic seaboard and the northern North Sea - sightings in the southern North Sea and English Channel are rare.'

In total, 14 cetacean - whales, dolphins and porpoises - species had been recorded in Norfolk, but most had only been seen a handful of times, often found stranded on beaches, like the 40ft sperm whale discovered dead on Old Hunstanton beach on Christmas Eve. By far the commonest species off Norfolk was the harbour porpoise.

The killer whale's diet included seals and so areas like Blakeney Point could provide rich hunting grounds but Dr Leech thought it more likely that the men had seen an individual which had drifted off course.

The submitted sighting records would be sent to the Norfolk Biodiversity Information Service, at County Hall, which held a database on all mammals seen in Norfolk and was a valuable resource for conservationists.

Dr Leech said he would also be passing the information to the Sea Watch Foundation and Marine Conservation Society.

Mr Dunmore said he had been unaware that his discovery was a 'first' for Norfolk but he has previous experience of making the natural history books. In 1970, while bird watching in Cornwall, he recorded the first-ever European sighting of the American Veery thrush.