STUDIES into the levels of chemicals in the rivers Tees and Skerne have revealed a trail of pollution stretching out into the North Sea.

Research by the Government's Centre for Environment Fisheries and Aquaculture, in 1999, said it was likely the brominated flame retardant chemicals (BFR) came from the Great Lakes chemical plant at Newton Aycliffe, County Durham, the only production site in the UK.

Despite this, the Environment Agency maintains that the contamination cannot be conclusively linked to the US-owned plant, which produced BFR for up to ten years, finally ending production in 1996.

Nick Cartwright, the agency's chemicals policy manager, said: "Certainly, there is a localised elevated level of these substances in sediments, but you cannot prove that it is caused by Great Lakes. It is historical contamination."

He said the build up could have occurred over decades and could also have eminated from more than one source.

"These chemicals get into the environment from all sorts of releases and that is the main concern. It is not easy to tie it up to one plant," Mr Cartwright said.

Research by Dutch scientists found high levels of two types of BFR in hermit crabs and starfish around the mouth of the Tees.

Much lower levels of pollution was found in invertebrates in other parts of the North Sea, suggesting the pollution came from the Tees.

Dr Bart Zegers, who carried out the study for the Netherlands Institute of Sea Research, said the major concern was that the chemicals could get into the food chain.

"In invertebrates we do not see a very high level of accumulation, or in fish; but, if you then look at seals and other sea mammals further up the food chain, it is stored in their fat and it does accumulate.

"Higher levels are being found in mammals, as well as humans, with levels being found in breast milk."

Concerns about the harmful effects of the chemicals on wildlife, which some fear could cause reproduction and growth problems, have led environmentalists to question the potential impact they could have on commercial fish stocks.

Pete Bowler, from Waterwatch, said: "It obviously demands further investigation, but if these chemicals have the potential to affect the breeding success of commercial species such as herring, the cost to the North Sea fishing economy could be huge."

He added: "The Environment Agency should use its powers to require the company to come forward with a clean-up proposal."

However, Mr Cartwright said there was little they could do about damage which has already been done

He said: "You would cause more environmental damage trying to clean something like this up, than if you left it."

Great Lakes no longer produces the Penta-brominated diphenyl ether (penta-BDE) or the Tetra-BDE chemicals.

But it does produce another brominated flame retardant compound - hexabromocyclododecane (HBCDD).

Mr Cartwright said the Environment Agency had identified this compound as being of potential concern and it is being assessed at European level.

He said: "Great Lakes has been cooperating in terms of providing information on releases of that substance to help with the risk assessment."

Bob Campbell, of Great Lakes, said: "If it is shown that HBCDD does present a particular risk of harm to wildlife or human health, we will do the right thing."

However, Mike Warhurst, of Friends of the Earth, called for a complete ban on the chemicals.

"Brominated flame retardants should be phased out because of their negative environmental impact," he said.

Mr Campbell disagreed, explaining the compounds used in televisions, foam, insulation and other everyday items, helped to save lives.

"Brominated flame retardants are there because they help protect life. Substantial numbers of lives have been saved and property protected because of these compounds."