Official says there have been multiple sightings of whale measuring at least 5m long

A humpback whale measuring at least 5 metres (16ft) has been spotted in the River Thames east of London.

“We are 99.9999% sure it is a humpback whale – there have been multiple sightings,” said a spokesman for the Port of London Authority (PLA), which oversees the river.

Sightings of the animal were reported over the weekend and its presence in the river was confirmed by British Divers Marine Life Rescue.

“It seems to be doing fine,” said Julia Cable, national coordinator for BDMLR. “We watched it doing normal diving and surfacing behaviour. It is surfacing once every five or six minutes, which is normal.”

A humpback whale that has been spotted in the Thames in an extremely rare London sighting. Photograph: Richard Banner/Twitter/PA

A humpback whale was spotted in the Channel last week, according to BDMLR, but it is not clear whether this is the same one.

Last year, a beluga whale, nicknamed Benny the Beluga, spent more than three months in the Thames. It is thought to have made its own way back out to sea in the new year. The PLA spokesman said of the latest sightings: “It is not Benny, it is not a beluga.”

Humpback whales, once prized by hunters for their blubber, can weigh up to 40 tons and reach 18 metres in length. They migrate tens of thousands of miles a year between the rich feeding grounds of the Arctic and breeding grounds in tropical waters.

While humpback whales are sometimes seen off the British coast further north, Cable said it was very unusual for one to be seen within the Thames estuary. “It’s very likely that it just made a navigational error,” she added.

In 2006, a northern bottlenose whale was spotted in the Thames in central London. On that occasion the whale, which was too weak to find its way back out of the river on its own, ultimately died as rescuers attempted to transport it back to sea.

In 2009 a humpback whale was found washed up on the shore of the Thames in Kent, having seemingly died from starvation.

Cable said there was no indication the whale spotted over the weekend was in any distress and it is hoped it would find its own way out.

She said the animal appears to be swimming as far upstream as Woolwich during high tide, then making its way further downstream during low tide. It was seen near Dagenham on Monday morning.