The commission accused the UK of failing to abide by a January ruling on the urban waste water treatment directive. It states that towns with a population of 15,000 or more must have the facilities to remove excessive nutrients from waste water.
The court found that 13 UK towns failed to meet the obligation. But in April, the government argued that six of the locations now have the facilities in place.
The commission urged the seven other towns, including Brighton, Margate, Broadstairs, Bangor and Donaghadee, to provide facilities "as soon as possible".
A Southern Water spokeswoman told Planning that an £80 million treatment plant for Margate and Broadstairs was completed last week. The firm has also tried for more than a decade to secure permission for treatment works that would serve Brighton and Hove, she added.
The company's £200 million scheme for Peacehaven was thrown out by the government in July because of problems with landscape and visual impact (Planning, 3 August, p3).
The Department of Regional Development insisted that Northern Ireland Water is investing nearly half a billion pounds on waste water treatment up to 2010.
A spokeswoman added: "We hope that we will not have to use money to pay fines that could instead be invested in improving water and sewerage services."