A last-minute deal between the UK and Spain – agreed just hours before Gibraltar was poised to become the only frontier marked by a hard Brexit – will allow for free movement between the British overseas territory and much of the EU. “Today is a day for hope,” Spain’s foreign minister, Arancha González Laya, said on Thursday as she announced that an agreement in principle had been reached. “In the long history of our relations with the UK, related to Gibraltar, today we’re facing a turning point.” As part of the deal, the British overseas territory located on the southern tip of the Iberian peninsula would be able to join EU programmes and policies such as Schengen with Spain acting as a guarantor, González Laya told reporters. “Schengen will be applied to Gibraltar, with Spain assuming responsibility as a member state,” she added. “This will allow for the abolishment of controls between Spain and Gibraltar.” Gibraltar’s airport and port will become the EU’s newest external border, with checks undertaken by the EU’s Frontex border agency. The arrangement will be in place for an initial four-year period. When pressed on whether this would entail the presence of Spanish security forces in Gibraltar – a point that had proved to be a major sticking point in the negotiations – González Laya said the technical details would be published in the new year. The agreement will now be sent to Brussels, where the European commission will enter into negotiations with London to turn it into a treaty, a process González Laya estimated would take around six months. In the meantime, she said Spain would work to ensure that mobility at the border would be “as fluid as possible”. The deal was hailed by the UK prime minister, Boris Johnson. “I wholeheartedly welcome today’s political agreement between the UK and Spain on Gibraltar’s future relationship with the EU,” he wrote on Twitter. “The UK has always been, and will remain, totally committed to the protection of the interests of Gibraltar and its British sovereignty.” Spain’s prime minister, Pedro Sánchez, said it marked the start of “a new era” that would allow for “the removal of barriers”. The Brexit deal announced on Christmas Eve between the UK and EU did not cover Gibraltar. Instead the fate of the territory was the topic of months of parallel negotiations that focused on preserving free movement across the shared border with Spain while steering clear of the centuries-old sovereignty dispute between London and Madrid. With just hours left before the UK was to leave the 27-member bloc, negotiations over the future of the territory had come down to the wire. “The final countdown” is how Fabian Picardo, Gibraltar’s chief minister, had started off the day on Twitter. Hours later, Picardo struck a cautious note. “I think it’s important to temper this moment with understanding that what we have is an in-principle agreement, not a treaty,” he told reporters. “When we go to the European Union, we need to ensure that the European commission shares our enthusiasm for this to happen.” He warned that the treaty would entail tough negotiations, including jurisdiction issues and import duties. “There will be complexity to come,” he said. “We may end up with a deal on the movement of people but not goods.” Picardo had long advocated for Gibraltar to join the Schengen area of abolished border controls – a move that would establish closer ties between the British overseas territory and the EU just as Britain left the bloc. On Thursday he defended the idea, even as he acknowledged that it would allow nationals from Spain and other countries that were part of the Schengen area to cross freely into Gibraltar while those arriving from the UK would be subject to passport controls. “This is the beginning of us building a stronger relationship with the European Union and with our neighbour Spain, in a way that doesn’t in any way cleave us away from the United Kingdom, which is our principal relationship,” said Picardo. In the 2016 referendum, 96% of voters in Gibraltar supported remaining in the EU. Still, the territory had come incredibly close to crashing out without a deal, noted Picardo. “The alternative is that, in about eight hours from now, without these arrangements, Gibraltar would be the only part of the European continent that would be suffering a hard Brexit.” The UK’s foreign secretary, Dominic Raab, hailed the UK’s “warm and strong relationship with Spain” in a statement, noting that the agreement would now seek to be formalised. “In the meantime, all sides are committed to mitigating the effects of the end of the transition period on Gibraltar, and in particular ensure border fluidity, which is clearly in the best interests of the people living on both sides.” Despite ceding Gibraltar to Britain in 1713, Spain has long sought to reclaim the tiny territory on the southern tip of the Iberian peninsula. On Thursday, Gibraltar and Spain stressed that the deal did not impinge upon their respective sovereignty claims. “There are no aspects of the framework that has been agreed that in any way transgress Gibraltar’s positions on sovereignty, jurisdiction or control,” said Picardo, echoing earlier remarks by González Laya.