Sweden's Commission on Marine Environment has warned that the Baltic Sea is in a "critical" condition and in danger of dying unless pollution from the Russian city of St Petersburg is drastically cut.
Half of the fish species in the Baltic are at levels below the critical biological level, while pregnant Swedish women are being warned not to eat herring - a staple diet - because of dioxins.
Untreated sewage flows straight into the Neva river -and from there to the Baltic
There is little dispute that St Petersburg - Russia's second-biggest city - is the Baltic's single biggest polluter, and behind many of the problems.
"We have come to the end of the road, concerning the sea," Hans Jonsson, chairman of the commission, told BBC World Service's Politics Of Water programme.
"We have huge problems with algae and plankton growth, with lack of oxygen," Mr Jonsson added.
The Baltic is the world's biggest brackish sea - a peculiar mix of sea and lake, salt water and fresh.
Its bed is a mix of ridges and troughs that trap pollutants, and starve fish and plant life of oxygen.
And because it only has a narrow outlet to the ocean - between Sweden and Denmark - the water takes 25-30 years to refresh itself.
Sweden is especially concerned because fishing is essential to the country's economy - although it involves only a few thousand Swedes directly, there are thousands more in associated businesses.
St Petersburg's sewage works are massively outdated and overburdened
But massive overfishing has decimated stocks, and pollution has meant they are unable to grow again.
"You can't throw out poison into a place that you produce food," said Andreas Jonsson, a coastal fisherman.
"On the Baltic side - Latvia and Russia - they don't clean their water from the towns."
The biggest of these towns is St Petersburg, a vast metropolis built on the Neva river.
Much of the city's sewage flows untreated into the Neva, and from there directly into the Baltic.
"You have a bit of a nutrient soup, which causes large growth of anaerobic bacteria as well as seaweed - which kills all life," explained Dietmar Litmanov, a member of the board of Greenpeace Russia.
The problems stem from the fact that St Petersburg is still waiting for a new municipal waste water plant to be completed - a project constantly put back due to costs.
"The government in St Petersburg - as well as the governments in all the Russian regions, and the central government - are prioritising the economy over the environment," Mr Litmanov stated.
"It is a story that has been repeating itself throughout the former Soviet Union, and unavoidably has led to major environmental catastrophes."
Environmental journalist Victor Teryoshkin told Politics Of Water that the collapse of communism had left St Petersburg with a "terrible quality of local sewage facilities".
Sweden's fishermen face an uncertain future
"30% of the total water dumped into the Neva river by St Petersburg companies and apartment buildings goes there unfiltered," he said.
"That means 300-400 cubic metres of water a day - this is a dangerous cocktail, filled with all kinds of heavy metals and human waste."
Mr Teryoshkin added that there had simply not been enough money to help the city clean up after 1991.
Although a new treatment plant is being built in the south-west of the city, the amount of investment needed continues to thwart the project's completion.
Attempts are being made to secure funding from European banks, but much more money is needed.
"We do have some serious problems here with waste water, and only 60% of the water dumped into the sea from St Petersburg has been filtered sufficiently," confirmed Alexander Ridko, head of the health and ecology commission at the St Petersburg legislative assembly.
Mr Ridko said that another problem was caused by illegal dumping of waste into the Neva.
But he added that because much of this was being undertaken by St Petersburg's main sewage treatment company, Vodokanal, there was little that could be done about it.
St Petersburg's residents have appalling quality water
"In the case of illegal spillages, there are cases of companies dumping waste products into the river," he said.
"In certain cases we can blame Vodokanal, the city's main water supplier.
"What we have to do is find the guilty parties, make them responsible, and punish them."
Vodokanal defended themselves, saying that they believed only 20% of the water flowing into the Baltic was untreated.
But they too pointed out that they did not have the resources needed.
"What's needed is money to build a sewage collector and complete the waste treatment plant," the company said.
"Unless we have the money and complete the construction, the dumping will continue."