The Government is to appeal against a European General Court decision to give Spain new rights to the waters around Gibraltar, Foreign Secretary William Hague has announced.
The European Union (EU) has allowed Spain to designate much of the sea surrounding the Rock of Gibraltar as a special environmental zone, to be maintained by the country.
But the UK has warned that the ruling could have a "detrimental environmental or economic impact" on the British Gibraltar Territorial Waters (BGTW).
In a written statement to MPs, Mr Hague said: "Following extensive consultation and on the basis of legal advice, the UK will be appealing against the court's decision.
"The Government has a clear responsibility to uphold the UK's position over the waters around Gibraltar. I want to reassure the people of Gibraltar that we will defend their interests on this matter."
The Spanish government wants to designate a Site of Community Importance (SCI), named Estrecho Oriental, in the waters around Gibraltar. But this new conservation zone overlaps the co-ordinates of the BGTW and entirely overlaps an existing adopted UK SCI, the Southern Waters of Gibraltar.
Mr Hague said: "The UK has sought to reverse the adoption of the Spanish site, first through diplomatic action with the European Commission and the government of Spain, and then through legal action. We continue to work closely with the government of Gibraltar to resolve this issue.
"The original legal case was launched by the UK in the European General Court on December 22 2009. This challenged the European Commission's listing of this site as a Spanish Site of Community Importance.
"The Government is clear that only the UK can submit sites covering BGTW. Spain cannot enforce the management or monitoring of the specific area in question.
"There is, therefore, a risk that, given its lack of access to this site, Spain will make decisions without accurate scientific information which could have detrimental environmental or economic impact on Gibraltar and BGTW."