Experts say they are unable to confirm a killer-whale sighting off the north Norfolk coast earlier this year.

If validated, the sighting, by two experienced birdwatchers, would have been the first ever from the Norfolk coast.

But county mammal recorder Dave Leech said that after assessing the men's written submissions and sketch, there was not enough information to be sure.

However, Dr Leech stressed that the pair's evidence had not been dismissed and the incident would officially be recorded on the database of the Norfolk Biological Information Service at County Hall as an 'unconfirmed' killer-whale sighting.

Dr Leech said he had discussed the evidence with other experts, including representatives from the Sea Watch Foundation.

They had all decided that the pair could equally have seen another middle-sized cetacean - the family to which whales, dolphins and porpoises belong.

'There is just not quite enough detail to say that it couldn't have been a white-beaked dolphin, for example. They are more common and there is slightly more likelihood that it could have been one of those,' said Dr Leech.

White-beaked dolphins had been recorded 23 times off the Norfolk coast, most recently in 2010.

'Cetacean ID is very, very difficult because you're usually getting a brief view of something very distant and you're not seeing the whole animal,' he added.

The birdwatchers had not recorded seeing distinctive patterning on the flank, or the pale patch behind the eye of a killer whale, and ascertaining the size of the mammal had also been difficult.

But Dr Leech added: 'However, it's very important not to lose this as a potential sighting.'

Over the coming months and years other cetacean sightings might support, or weaken, the record and it was therefore an important in building a clearer picture.

The sighting, on February 9, was made by Giles Dunmore, from Beeston, near Sheringham, a long-serving former Norfolk bird recorder, and fellow birder Phil Vines.

They were scanning the sky and water with telescopes for incoming morning birds when Mr Dunmore, who had seen killer whales off Tenerife and Alaska, spotted a disturbance far out to sea. He described seeing a black hump and a 'great big, black dorsal fin'. Mr Vines also saw the animal.

The majority of killer-whale sightings in UK waters are on the Atlantic seaboard and the northern North Sea.

Male killer whales, or orcas, can grow up to 9.5m long (31ft). Male white-beaked dolphins reach up to 3.1m (10ft 2in).