According to records they have never been seen before in the River Great Ouse, but photographer Kevin Elfleet managed to capture these images of common dolphins feeding in the waters around King's Lynn.
In fact their appearance around Norfolk is so rare there have only been 13 sightings since 1943.
More commonly seen around Cornwall, Devon and the west of Scotland it is thought that a pod of around four or five common dolphins found their way up the tidal estuary from the North Sea.
Mr Elfleet initially thought they were harbour porpoise when he took the photos last week, but his suspicions led him to contact Carl Chapman from Northrepps, cetacean recorder for Norfolk.
'I'd had a look on the internet because the dorsal fins looked different to the harbour porpoise but it wasn't until I emailed some pictures to Carl that he was able to confirm they were dolphins,' he said.
'They were between the two bridges over the A47 south of King's Lynn and I could see they had fish in their mouths. It was quite interesting.'
Mr Chapman, who also runs Wildlife Tours and Education, said the photos were excellent quality which gave him enough information him to easily identify the dolphins.
'It is normal for harbour porpoise to come up river at this time of year and I had seen a video of them last week but something was not quite right,' he said.
'Then when I saw Kevin's photos I could see a more erect dorsal fin, not the triangular one of the harbour porpoise, and the surfacing movements were different. It also shows the beak, a harbour porpoise does not have one as such, just a nose. You can also see a demarkation line on the body where it is light and dark and that is a dolphin marking.'
The last common dolphin was seen at Sheringham last year but they have never been recorded in the River Great Ouse before.
Mr Chapman believes they have followed some fish and looked healthy, indicating that they have stayed in the salty tidal estuary and not gone into fresh water which can cause their skin to blister.
'They are quite friendly and will take a ride in the wake of a boat so if anyone is around the River Great Ouse they should look near moving boats to see if they are still around.'
* Have you seen any unusual wildlife? Email kathryn.cross@archant.co.uk
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here