A whale has been spotted swimming in the River Thames, following on from the visit of "Benny the beluga" a year ago.
The mammal was first spotted over the weekend, with sightings of it either side of the Dartford Crossing.
British Divers Marine Life Rescue (BDMLR), who saw it surfacing off Greenhithe on Sunday, said it was "definitely a humpback" and did not appear to be in any distress.
Ships have been advised to proceed with caution through the area.
"Benny the beluga" spent about three months in the busy waterway at the end of last year, although the new whale is not the same species as it is dark coloured and has a dorsal fin.
Julia Cable, the BDMLR's national coordinator, said a group of volunteers had observed the mammal surfacing repeatedly over a three hour period.
She told the BBC that it "seems to be fine" and was likely to have arrived because of a navigational error, possibly during the recent high spring tides.
"It really shouldn't be there but hopefully it'll find its way out," she said.
The Port of London Authority (PLA) said "numerous sightings" had been made from passing boats either side the Dartford Crossing, while there have also been reports of it off Rainham and Erith Pier.
A spokesman said people who had seen it had estimated it was five or 10 metres in length.
He added that its behaviour would be monitored by experts while it remained in the river and ships had been told to "proceed cautiously" when travelling through the area.
"Essentially it is a natural animal in a natural environment and we wouldn't intervene with that at this stage," he said.