Spain will seek to take its dispute with the UK over Gibraltar to the UN - with the support of Argentina.
Spanish foreign minister Jose Garcia-Margallo is expected to propose that both countries present a "united front" over Gibraltar and the Falkland Islands, El Pais reported.
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", the newspaper said.
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.
The country'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 UN's General Assembly or the International Court of Justice at The Hague, diplomatic sources reportedly told El Pais.
A 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 British warship will visit Gibraltar later this month, amid increased tensions in the overseas territory with Spain, in what the Ministry of Defence (MoD) has said is a "long-planned" training deployment.
Political tensions in the region have flared up after the British territory began work on a concrete reef in the Mediterranean which Spain claims will destroy fishing in the area.
Madrid responded by beefing up border controls and suggesting that a 50 euro (£43.30) fee could be imposed on every vehicle entering or leaving the outpost through its fenced border with Spain.