The sighting of a humpback whale in the North Sea off the Norfolk coast is the first on written record, experts said.
The whale spent most of Wednesday morning swimming about two miles out to sea between Winterton and Horsey.
Carl Chapman, cetaceans recorder for Norfolk, said having examined records dating back to the 1700s it was a first "to record a humpback off our coast".
First seen on Tuesday and surrounded by gannets, it has stayed because of a "good supply of fish", said Mr Chapman.
A regional co-ordinator for the Sea Watch Foundation, he added: "There's been an increase in the number of humpback whales around the world in the past 20 years since the hunting of them stopped.
"It was only going to be a matter of time before we had one off our coast, but to have one in our waters is a real thrill and it's possible it could over-winter here."
He said:"It was an amazing thing to see. The first thing I saw was the blow as it surfaced to breathe. It stayed in one area for quite some time surrounded by gannets.
"It then swam towards Sea Palling before what appeared to be turning out to sea. But for it to come back today it's clearly found a good food source. Nobody expected to see it again."
Other whales recorded in Norfolk by the Norfolk Biodiversity Information Service include the fin whale, minke whale, sperm whale and long-finned pilot whale.
Gemma Walker, from the Norfolk Wildlife Trust, said she was so determined to see the humpback she just put a coat over her pyjamas.
"I live at Winterton and thought when am I ever going to see one in my lifetime off the Norfolk coast - it was an opportunity not to be missed.
"I've been to Australia to see the humpback migration and didn't see a thing, I never thought I'd see one on my doorstep."