The scrapping of ships from a so-called American ghost fleet on Teesside could cause irreversible damage to a nearby bird sanctuary, it is claimed.
The first two ships have already left the US
As two more vessels are due to leave the US for Britain, the Royal Society for the Protection of Birds (RSPB) is calling for them to be turned back.
Two ships are already being towed to Teesside to be scrapped at Able UK's Hartlepool yard.
A further two were due to leave on Sunday on the 4,000 mile journey from the James River in Virginia.
Environmentalists say the vessels, which are contaminated with asbestos and other chemicals, could cause an ecological disaster.
Hartlepool-based Able UK has signed a £16m deal to scrap the vessels, creating 200 jobs.
The company has assured the UK government and environmental agencies that there is no risk to the public or the environment with the decommissioning work.
But the RSPB has called for the ships to be refused entry to the UK until the possible impact on important wildlife havens can be assessed.
Able UK's site is close to internationally important wildlife sites at Seal Sands and Teesmouth.
Nick Mason, conservation manager for the RSPB, said: "Our understanding is that there is no safe place to hold these ships on Teesside.
"Until there is a cast-iron guarantee that this work will have no adverse impacts on the internationally important wildlife of the Tees estuary, we do not want to see any more ships set sail.
"We would remind everyone involved in this issue that the wildlife and habitats of Teesmouth and the Cleveland coast are designated as a special protection area under the European Birds Directive."
The Tees estuary contains a wide range of internationally important wildlife habitats which are protected by tough UK and European directives.
The estuary is especially important for the more than 20,000 wildfowl and wading birds that use the area every year.
Mr Mason added: "Any contamination of the fragile and sensitive estuarine habitats by oil, PCBs, or other pollutants could have a major impact on the wildlife of the area.
"Our current assessment of the situation is that the plans to scrap the ghost fleet could impact on the wildlife of the Tees.
"We are very concerned that ships are already on their way and others preparing to set sail, before a full consideration of the environmental impact has been carried out here in the UK."
Able UK says it has taken steps to ensure that the environment is properly protected.
A spokesman said: "These vessels have been subject to far more stringent checks by far more agencies than probably any other ships which have come into the Tees."