The British government is preparing to lodge a formal complaint with Spain over Gibraltar. Spanish military aircraft have twice flown over the Rock in the past ten days: once on 27 September, reportedly while a commercial British Airways flight was taxiing on the local airport’s runway, and then again on 30 September. These flyovers (thought to have been by Spanish Air Force cargo aircraft) might seem harmless – but the British government disagrees.

The timing of the flyovers couldn’t be worse. They come shortly after the latest round of talks in Brussels between Spain, Gibraltar, the UK and EU about the Rock’s post-Brexit status. Of central importance to these negotiations are the questions of Gibraltar’s airport, airspace and its land border with Spain (now also a UK-EU frontier).

Spain wants ‘joint use’ of Gibraltar’s airport – but for Damid Lammy, the UK’s foreign secretary, the practical implications of such an arrangement are far from clear. Somewhat ridiculously, in last month’s talks, the various parties discussed where security guards should stand in the airport and even what colour uniforms they should wear. Yet these apparently trivial points are symbolic of a much deeper dispute – one that ultimately rests on the phrasing of a 300 year-old contract.

All parties now agree that passport control should take place in the airport, but Lammy is wary of such demands. At last month’s meeting, he reiterated that the independence of the British RAF base, which operates the territory’s runway, must not be compromised by any agreement on airport use and border control. Defence is one of only two areas in which Gibraltar is controlled from London, the other being foreign policy; in all other respects, the British Overseas Territory is self-governing.

Lammy has taken over from where his predecessor Lord Cameron left off in April. After that month’s discussions, Gibraltar’s chief minister Fabian Picardo said that all parties were ‘within kissing distance’ of a permanent agreement on issues such as the airport, goods and mobility. But after Lammy’s visit to Brussels last month, it looks like progress has slowed once again.

Part of the problem is that Spain wants much more than just joint control of Gibraltar’s airport. Citing the UN’s definition of the Rock as a ‘non-autonomous territory that must be subjected to a process of decolonisation’, the Spanish government ‘wishes this territory to be returned to it’. In a speech to the UN General Assembly last month, Pedro Sanchez, Spain’s Socialist prime minister, reiterated that position, saying that any post-Brexit agreement on the Rock must ‘fully respect UN doctrine’. Spain also claims that key aspects of the territory were excluded from the deal in which the Rock was ceded to Britain over three centuries ago.

Signed in 1713, the Treaty of Utrecht transferred ‘the city and castle of Gibraltar, together with its port, defences and

fortresses’ from the Spanish to the British’. But, says Spain, that document made no reference to the isthmus, the thin strip of land that connects the Rock to Andalucia – on whch the territory’s civilian and military airports stand – nor to the seas around the territory or the airspace above it. Therefore, according to the Spanish government, ‘the occupation of the isthmus [by Britain] is illegal and contrary to international law’. This centuries-old contract is also the reason why the UK is unlikely to receive a sympathetic response when it complains about the rogue cargo plane: Madrid will simply respond that it was flying through Spanish skies.

One might expect Spain to have pounced upon any opportunity to expose what it sees as the Treaty’s disastrous omissions. But in 1966, when the UK invited Spain – then ruled by dictator Francisco Franco – to resolve the issue of Gibraltar’s status at the International Court of Justice, the latter refused. Were the Spanish concerned that their archaic arguments about the isthmus wouldn’t survive scrutiny in The Hague?

If so, this concern seems to have been justified. In 2016, the international Court of Arbitration for Sport ruled that ‘under public international law, the sovereignty of Gibraltar is clearly British’. Picardo, Gibraltar’s chief minister, has been equally emphatic: ‘The concept of joint sovereignty or any dilution of our sovereignty is a dead duck’, he said in 2019.

That same year, however, I wrote an article arguing that a post-Brexit Gibraltar should reconsider joint sovereignty, mainly to keep an open border with the EU. Such an arrangement, I suggested, would speak to the facts that 96 percent of Gibraltarians voted to remain in 2016 and around 15,000 workers cross the border every day, almost 10,000 of whom are Spanish. But in a published response, Gibraltar’s deputy chief minister Joseph Garcia argued that this solution only makes sense if you approach ‘the question as one of logic or arithmetic’. It fails to appreciate what Garcia called the ‘existential’ aspect of being Gibraltarian – ‘our way of life, the elements that make us a people’.

Certainly, Spanish claims to the Rock don’t reflect the interests of Gibraltarians. In two referendums, held in 1967 and 2002, they overwhelmingly rejected the prospect of control from Madrid, opting in both cases to remain as a British Overseas Territory. Franco was so unsettled by that first vote, and the strongly pro-British Constitution in which it resulted, that he shut the Gibraltar-Spain border in 1969; it wasn’t fully reopened until early 1985.

The dispute that Lammy has just entered is not primarily about Brexit, although of course it has been complicated by that vote. Gibraltar’s airport, located as it is on that crucial strip between the Rock and Andalucia, is the modern focal point of a 300 year-old argument. And so is that cargo plane.