A dolphin is being monitored by a marine rescue charity after it swam miles up a Welsh river and into English waters.

Marine experts think the common dolphin, which is usually more at home in the deep seawater of the Bay of Biscay off France, must have been chasing fish up the Dee in north Wales.

The disoriented creature was first spotted by the public in Connah's Quay docks in Flintshire, north Wales, on Monday but then swam further up river to Saltney.

It carried on upstream and was most recently seen near Chester racecourse.

Locals have been tweeting pictures of the mammal jumping and flipping out of the water.

Stephen Marsh, operations manager at British Divers Marine Life Rescue, said they are keeping a close eye on the dolphin in case it gets stranded on a sandbank.

Marsh said the creature started heading back out to sea but today it had returned to Chester.

He said: "It's probably been chasing fish in from the sea and then got caught up in the tidal system."

He said tides at this time of year can be very high and very low and that is probably confusing the dolphin, which is used to much deeper water.

Marsh said such an event is quite rare, adding that they would step in to take it back out to sea if it does get into trouble.

However, he said it looks healthy and they hope it will find its own way out.

A member of Environment Agency staff doing maintenance work on the Dee spotted the dolphin in Chester and took a photograph as it jumped out of the water .

A spokesman for the Environment Agency said: "It's likely that the dolphin entered the River Dee searching for food.

"Our rivers are the healthiest that they have been for over 20 years. Because of this we've seen an increase in the numbers of wildlife including otters and salmon returning to our waters."

• This article was amended on 22 August 2013. An earlier version said that Saltney is in Cheshire. Saltney is, in fact, in Flintshire, north Wales, but on the border with Cheshire.