The estuary is home to a variety of birds

Experts say the oil slick could have been disastrous for the area, which is world-famous for wildlife.

It entered the estuary from Thornleigh Cut, which drains the Sealand area of Chester, but experts said most of the spill was washed away by tides.

Alan Winstone, North Wales Coast Environment Manager, said a clean-up operation was under way.

"We are closely monitoring the situation," Mr Winstone said.

"It seems that the impact of the spill on the estuary has been limited by our quick response and the dispersion of the oil by the tides.

"We are continuing our efforts to trace the source of the spill, which could have had a disastrous impact on the wildlife of this very special estuary.

"We don't know yet whether this was an accidental or deliberate spillage."

The Dee Estuary is a haven for wildlife, including hundreds of thousands of birds.

The freshwater Dee, above Chester weir, has not been affected by the spill which was discovered on Wednesday.

Anyone with information is asked to call the 24-hour Environment Agency Wales hotline on 0800 807060.