The legal study of the Strait of Gibraltar and the rights involved is quite complex due the historical background, geo-strategically and political interests of Spain and Crown Colony Gibraltar (CCG)and Morocco . We will leave apart the clash between the principles of self-determination or territorial integrity – by the way, the UN already said in 60´s of last century the last one was dominant – and its political consequences and will focus just in the Law of the Sea aspects.
Apart from the general application of the transit passage established for any strait in art 37 of United Nations Convention on the Law of the Sea (UNCLOS), which entered into force in 1995, also called the Law of the Sea Convention there are different interpretations of the extension of coastal jurisdiction. Spain does not recognize the UK’s sovereignty over the entire CCG and especially British jurisdiction over part of the isthmus joining it to Spain with the airport constructed on it, and consequently does not accept that the CCG has a right to any territorial waters according to the wording of the legal title, the art. X of the Utrecht Treaty from 1713:
“The Catholic King (Philippe V) does hereby, for himself, his heirs and successors, yield to the Crown of Great Britain the full and entire propriety of the town and castle of Gibraltar, together with the port, fortifications, and forts thereunto belonging; and he gives up the said propriety to be held and enjoyed absolutely with all manner of right for ever, without any exception or impediment whatsoever”.
The territory ceded by under Treaty is exposed in an extremely clear manner in the text of the Treaty. However, the fact is that a simple glance of the political and geographical map of this area shows how the territory occupied by United Kingdom is not correlated with the area of the Treaty in 1713 with the argument that the Treaty says nothing about reclaimed land, or territorial waters. This fact, together with a similar, but not exactly equal case of the of the Ceuta, Melilla, Peñón de Vélez de la Gomera, Alhucemas and the Chafarinas Islands claimed by Morocco, supports the CCG position.
The UK established unilaterally the 3 nautical miles jurisdiction on territorial sea according to historical tradition and since 1982 according to the application of the UNCLOS Convention on the Law of the Sea (which set a new standard of 12 nautical miles). Most countries, including the members of the European Union, are signatories to the Convention. However it s no clear if said Convention applies in equal circumstances to Gibraltar as a territory under decolonization in UN terms and international Law. That is why Spain added a comment regarding Gibraltar at the time of its adherence to the Convention expressing the exception of the Gibraltar as subject in decolonization process according UN resolutions in force. It did, however, record the view taken by Spain on this point.
The overlapping of the territorial waters in the Gibraltar Strait have caused numerous conflicts between the UK, Gibraltar, Spain and Morocco (fisheries, navy, smuggle, ect.) Unfortunately, and despite the media vigorous articles Brexit will not technically change the legal status of the Law of Sea with respect Gibraltar waters.
Only a triple engagement between all the coastal States will bring order to said situation.