The UK is under pressure to better protect harbour porpoises after an EU court ruled that it had failed to provide enough sites for the sea mammal.

The European Court of Justice has ruled the UK has failed to propose a sufficient number of special areas of conservation (SACs) for harbour porpoises under EU legislation in waters off the UK coastline.

There are seven areas in English, Welsh and Scottish waters designated to protect porpoises, the smallest cetacean found in European waters, six of which were created after previous action by the European Commission.

But environmentalists said scientific evidence showed more are needed, particularly in Scotland.

Environmental law charity ClientEarth’s wildlife conservation lawyer Tatiana Lujan said the UK had “dragged its heels and failed to comply” with rules to protect harbour porpoises.

With just one conservation site in Scotland, “harbour porpoises in Scotland’s eastern and northern waters simply do not have enough areas to catch food, breed, rest and communicate safely”, she said.

“We hope to see a comprehensive set of UK sites for harbour porpoise protection as soon as possible, because they have been left without proper legal protection for far too long.”

Wildlife charity Whale and Dolphin Conservation (WDC) said harbour porpoises were Europe’s smallest cetacean and they “live fast and die young”, at just 12 years on average.

Recent evidence shows they may be struggling to reproduce successfully because of high levels of pollution, and they die in high numbers in fishing nets and are disturbed by ocean noise including wind farm construction.

While efforts to designate the existing six sites is “welcome”, more needs to be done, WDC said.

More protected areas need to be created and those that are already designated need stronger measures to protect harbour porpoises within them.

The charity’s policy manager, Sarah Dolman, said: “WDC strongly welcome those porpoise SACs that have been designated in English, Welsh, Northern Irish and Scottish waters.

“We would like to see clear, robust and precautionary conservation objectives and effective monitoring and management at the site level to ensure maintenance of favourable conservation status.”

A spokeswoman for the Environment Department (Defra) said: “In the last year alone, we’ve taken significant steps to protect harbour porpoise in our waters, including creating six new special areas of conservation for this important species.”

She said the UK had taken other steps to protect sea life including proposing 41 new marine conservation zones around the country.

“We are committed to protecting our seas and marine life, and will now carefully consider the court’s ruling,” she added.