The whale which was seen in King's Lynn today was a fin whale, experts say.
It is believed to be only the ninth time one of the creatures has been seen in Norfolk in more than 150 years.
MORE - pictures and video footage of the amazing sighting
Fin whales are the second largest animal on our planet, with adults reaching lengths of more than 25m and weighing 75 tonnes.
The whale seen in the Ouse - estimated at around 5m - was a juvenile.
The Seawatch Foundation said the creature, which was sighted in the river upstream of the town centra around 8am, looked 'disoriented' and showed fresh scars on its back, flank and head.
'It would be more typical of a Minke Whale to enter the River Great Ouse but historically there have been two other records of Fin Whale within the river.
'Therefore it's not so outlandish that this individual that can grow to be the second largest animal on the planet came to visit King's Lynn today. The latest good news is the whale was last seen heading back out to sea' Sea Watch regional coordinator for Norfolk, Carl Chapman said.
Sea Watch chair of trustees, Robin Petch, has alerted British Divers Marine Life Rescue and they are sending a team to assess the situation.
'With the help of the British Divers Marine Life Rescue we really hope this fin whale is going to be able to swim back to the open sea as the tide is going out and time is ticking', said Chiara Giulia Bertulli, sightings officer for Sea Watch.
While the whale's entire body could not be seen in the coloured water of the tidal river, its dorsal fin can clearly be identified.
The fin whale, Balaenoptera physalus, has a sleek and streamlined body with a dorsal fin which tends to be taller and set farther forward on the rear of its body compared to blue whales, and which sets further back and rises at a shallower angle than those of other species.
The creatures inhabit the North Atlantic, North Pacific, Mediterranean, South Pacific, Indian and both Arctic oceans.
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