|The HMS Westminster leaving Portsmouth for Gibraltar|
As we have argued, neither a challenge on free movement rules nor one on "proportional" border checks carries a guarantee of success due to the ambiguity of EU law. Equally, the Spanish feel they have a strong legal case against the artificial reef based on the very specific wording of the 1713 Treaty of Utrecht - meaning a retaliatory case is not out of the question.
An alternative could be an individual or collective challenge by Gibraltarians (or even Spaniards working in Gibraltar) to the Strasbourg-based European Court of Human Rights, but that could take a long time.
Since then, tensions have escalated with the dispatching of several Royal Navy vessels to Gibraltar (reportedly as part of a long planned manoeuvre) and claims in the Spanish media that the country could from a united diplomatic front with Argentina, which of course has its own axe to grind with the UK. Although we think a diplomatic solution is still the most likely outcome, if none of the sides are willing to back down the UK may be forced to actually initiate legal proceedings, most likely under a 'fast-track' arrangement, as it will by then not have many other practical options.
While initiating a legal challenge may itself force all the sides to resume negotiations, should Madrid still not back down and should the ECJ rule in its favour, this dispute may have more fundamental repercussions on the UK's future in the EU.