TESTS have been carried out after a stream in the Yorkshire Dales turned "luminous green" overnight.

Experts believe a dye used to trace water sources caused the discolouration of Cogden Beck, near Reeth, in Swaledale.

Local residents were initially concerned the stream might have been polluted by toxic chemicals, but the Environment Agency later stressed that the dye was not harmful to humans or animals.

The green water was spotted in the stream which flows off Grinton Moor and into the River Swale on Wednesday morning.

Green water was later noticed several miles away in the Swale in Richmond.

Des Coates, head gamekeeper with the Grinton Estate, noticed the discolouration early on Wednesday morning and the colour remained in the water several hours later.

He said: "It happened overnight. It's a luminous green - the photographs don't really do it justice.

"It's bubbling up that colour from the spring. It must be some kind of chemical - it's such an unusual phenomena and I've no idea what's caused it."

Worried that it might be toxic, Mr Coates informed farmers in the area who used water for the stream for their animals.

He said staff from the Yorkshire Dales National Park Authority collected samples of the green water on Wednesday morning.

The Environment Agency said tests showed the green colour was caused by a dye used to detect water sources and it appeared to be coming from an underground spring.

A spokesman said it was not known who used the dye or why, adding: "We are confident this is not causing a problem."

Yorkshire Water said they were notified by the Environment Agency about the issue, but advised officials that the discolouration was not caused by their staff and the colour was not having any impact on services.

The stream crosses underneath the road on the Cote de Grinton - a hill climb on the route of the Tour de France when the first stage was held in Yorkshire last year.