Spain will seek to take its dispute with the UK over Gibraltar to the United Nations with the support of Argentina, according to Spanish media.
Spanish foreign minister Jose Garcia-Margallo is expected to propose that both countries present a “united front” over Gibraltar and the Falkland Islands, the newspaper El Pais said.
Mr Garcia-Margallo will sound out his Argentinian counterpart, Hector Timerman, during a meeting in Buenos Aires next month as he prepares for a “180 degree turn in policy towards the colony”, it is claimed.
Such a move would be seen as an escalation in the diplomatic tensions surrounding the overseas territory.
Argentina is on a two-year term as non-permanent member of the UN’s Security Council and could potentially use its position to include discussions over Gibraltar on the agenda.
Argentina’s president, Cristina Kirchner, used a meeting of the council this week to renew demands for talks over the sovereignty of the Falklands.
Spain is also considering the possibility of raising the matter of Gibraltar at the General Assembly or the International Court of Justice at The Hague, diplomatic sources reportedly told El Pais.
A UK Foreign Office spokeswoman said: “Our policy on Gibraltar has not changed and is consistent with our policy on other overseas territories.
“Self-determination matters more than territorial integrity.
“The people of Gibraltar have repeatedly and overwhelmingly expressed their wish to remain under British sovereignty.”
A warship will visit Gibraltar this month, amid increased tensions in what the Ministry of Defence said is a “long-planned” training deployment.
“We are studying taking the matter to the UN and these are all options that are being considered,” a Spanish source said, talking of the El Pais report.
“The minister is travelling to Argentina in September and plans to exchange ideas over the matter,” he added.
Prime Minister David Cameron and his Spanish opposite number Mariano Rajoy had agreed to try and calm tempers over the disputed territory, though both sides have been reluctant to back down.
Gibraltar, the tiny rocky promontory near the southern tip of the Iberian Peninsula, has been a source of on-off tensions since Spain ceded the territory to Britain in the Treaty of Utrecht 300 years ago. They flared up after the Gibraltar began work on a concrete reef in the Mediterranean which Spain claims will destroy fishing in the area.
Madrid res-ponded by beefing up border controls and suggesting that a €50 (£43.30) fee could be imposed on every vehicle entering or leaving the outpost through its fenced border with Spain.
In an interview on Spanish television at the weekend, Mr Margallo said the fee would not be imposed on workers who frequently cross the border for their jobs and pledged aid to the fishermen whose livelihood is being hit by the reef.
Travellers as well as residents of both Spain and Gibraltar continued to endure long queues at the border over the weekend because of the Spanish authorities’ increased checks on vehicles entering and leaving the territory.
Last week, Ms Kirchner renewed her claim to the Falklands at the Security Council, calling for the international community to enforce the 1965 resolution that Britain and Argentina should negotiate.
Islanders voted overwhelmingly in favour of staying British when a referendum was held in March, but Argentina rejected the legitimacy of the poll, calling it a “stunt”. The country said the “occupied” islands will be back under Argentine control within 20 years.