Scotland has come under fire from campaigners for lagging behind the rest of the UK in efforts required by EU law to protect the harbour porpoise.
The UK government has announced proposals for five potential special areas of conservation (SACs) to protect the species in Welsh, Northern Irish and English waters.
However, Scottish ministers recently shelved plans for four sites north of the border.
Seas around the UK are home to some of the highest densities of harbour porpoises in Europe. But populations are declining due to a range of threats that affect their ability to catch food and breed.
The species is protected under the EU Habitats Directive, which means the UK government must designate SACs in places that are known hotspots and considered vital for survival and reproduction.
Conservationists have welcomed the proposals, but say Scotland could jeopardise the nationwide scheme.
Pine Eisfeld-Pierantonio, conservation and policy officer for the charity Whale and Dolphin Conservation, said: “WDC greatly regrets the decision by the Scottish Government, which seriously undermines the whole UK process and the achievement of an ecologically coherent network of marine protected areas.
“The loss of the four Scottish sites also means that at UK-level sufficiency of marine protected areas will not be achieved.”
The organisation is calling for ministers to “provide a transparent process and timeline to designation” for harbour porpoise SACs.
The UK has already received a warning from the European Commission, threatening legal action over its failure to protect the species.
ClientEarth wildlife lawyer Catherine Weller described the latest proposals as “a step in the right direction” but said Scotland’s delay could undermine any positive effect. “If Scotland persists in delaying then it remains at odds with its EU obligations,” she added.
A spokeswoman for the Scottish Government said it “remains fully committed to having harbour porpoise special areas of conservation in Scottish Waters, where they are fully justified and supported by the evidence”.
She said proposals for Scotland “did not fully meet the scientific requirements”, but a new selection process had begun and would “progress as quickly and methodically as possible”.