The area was designated an SSSI earlier this year

A clean-up operation is continuing on Teesside after a large area of marshland near Seal Sands was contaminated by a spillage of hydrochloric acid from a chemical plant.

A spokesman for the Royal Society for the Protection of Birds said finding out what the effects had been would be a "long-term process".

It is feared that birds that come to the marshes to feed could have been affected by eating contaminated worms.

RSPB spokesman David Hirst said the exact damage caused by the acid on vegetation and invertebrates on the marsh was difficult to judge.

The BBC's John Thorne: Ecologists have been monitoring the polluted water

He said: "To look at, the marsh seems no different, but it is what is going on underneath the surface that is important.

"Although thankfully there seems to have been no mortalities yet among the birds, the vegetation and invertebrates are affected and so therefore are the birds."

'Cornerstone of wildlife protection'

Mr Hirst continued: "Finding out exactly what damage this leak has caused will be a very long-term process over a period of months, but it is vital that the area is returned to its previous state.

"Sites of special scientific interest may not sound very exciting but they are the cornerstone of wildlife protection."

The area is used by hundreds of wading birds as a roosting and feeding patch.

A spokeswoman for the Environment Agency said exact details of how and when the leak occurred would be investigated next week.

A spokesman for Tioxide Europe said: "Acidity levels are decreasing and the pH level has risen from two to three overnight which is a considerable improvement.

"More samples will be taken and tests carried out on sediment from the area as the pumping operation continues."