The EU executive said it was taking Britain to court after rainstorms repeatedly caused sewers to flood into rivers in London and Whitburn, in the north-east of the country.
The Commission also said it was sending Spain a final written warning over sewage spills in the area of Playa de la Motilla, a beach near Valencia.
"I urge Spain to clean up its act swiftly," EU Environment Commissioner Stavros Dimas said in a statement. "Should it not, the Commission will consider asking the court to impose fines."
Health threat
Faecal bacteria and viruses in sewage overspills are a danger to swimmers using nearby beaches, while excessive nitrates and phosphates from detergents can cause algae to grow rampantly and choke off other forms of life.
The European Court of Justice ruled in 2007 that Spain had failed in its obligation to treat appropriately the waste water from the cities of Sueca, Benifaio, Sollana, Almussafes and other coastal towns near Motilla beach.
"While the agenda for the works remains unclear, waste water is being released into the environment," the Commission said.
The EU executive said the 1991 urban waste water directive did allow Britain some flexibility on spillages from its combined sewers during rainstorms, but it had gone too far.
Overflowing sewers helped push British beaches down the EU cleanliness rankings for the second year running in June.
Pressure for more investments
The case, also at the European Court of Justice, could create pressure for more investment by London's Thames Water, part-owned by Australia's Macquarie, and Northumbrian Water Group, which controls Whitburn sewers.
British authorities will point to €2.7 billion of investment in the sewage systems of England and Wales since they were privatised in 1989.
A further €1.01 billion is planned for improving combined sewage outlets and €2.8 billion is earmarked for stormwater tunnels beneath London.
(EurActiv with Reuters.)
