A high court judge has ruled a local authority should have insisted on an environmental impact assessment before granting planning permission for work at a quarry.
The High Court in Manchester heard Cornwall Council had acted unlawfully.
Shire Oak Quarries wants to mine rock from Dean Quarry, St Keverne, Cornwall.
It is hoped the rock will be used in a tidal energy project in Swansea Bay. Cornwall Council said it was discussing its options.
Shire Oak Quarries has permission to extract stone from the quarry until 2035 and has said the only viable way of getting it to west Wales is by transporting it by boat.
A full application for two jetties and a protective breakwater is yet to be submitted.
However, opponents fear the quarrying could damage the environment and local economy.
Christopher Tofts, from Stephens Scown solicitors, who represented the claimant - a dairy farmer who lives nearby - said the objection related to fuel tanks and other facilities needed to bring the quarry into operation.
A spokesman for Shire Oak Quarries said: "We are disappointed with the judgement and now await the council's next steps in relation to determining our application for ancillary works."
The council said it acknowledged the judgement and added it would be discussing options with its legal advisors.