Gibraltar: timeline of British-Spanish tensions

Tensions have escalated between Britain and Spain over the past few weeks over the rocky outcrop of Gibraltar.

Spain ceded the territory to Britain in 1713, but has long argued that it should be returned to Spanish sovereignty. London says it will not do so against the wishes of the roughly 30,000 Gibraltarians, who are staunchly pro-British.

Tiny Gibraltar enjoys a strategic location at Spain's southern tip, poised at the gateway between the Mediterranean and Atlantic Ocean.

Fishing rights have long been a source of discord between "the Rock" and Madrid. Gibraltar has accused Spain of deliberately creating border hold-ups in retaliation for its construction of an artificial concrete reef.

Gibraltar says the reef is aimed at boosting fish stocks, but Madrid claims it is a bid to impede Spanish fishing vessels.

A timeline of recent relations between Britain and Spain over Gibraltar:

- September 18, 2006: Britain, Spain and Gibraltar sign several landmark agreements aimed at ending years of bitter quarreling about the status of the territory, paving the way for commercial flights from Spain to Gibraltar and ending a long-running pensions dispute.

- December 16, 2006: Passenger flights between Gibraltar and Spain start up again, ending an aerial "cold war" dating back to 1954.

- July 21, 2009: Foreign minister Miguel Angel Moratinos becomes the first Spanish minister to set foot in Gibraltar since it was captured by English troops in 1704.

- February 1, 2012: Spain's new conservative government, led by Mariano Rajoy, calls on Britain to agree to talks on Gibraltar's sovereignty.

- May 16, 2012: Spain's Queen Sofia cancels a planned trip to London to attend Queen Elizabeth II's diamond jubilee celebration lunch because of tensions over Gibraltar.

- May 24, 2012: Spain warns that it will continue to defend its fishermen in the contested waters around Gibraltar following a clash between police boats from the two sides over fishing rights.

- November 15, 2012: Spain and Britain summon each other's ambassadors over what London calls a series of "serious incursions" in the waters off Gibraltar.

- June 25, 2013: Gibraltar claims Spanish police shot at a jet ski in its waters. Spain flatly denies the accusation and criticises Britain for giving credence to the rumour.

- July 3, 2013: Gibraltar complains that four Spanish air force jets entered its airspace without permission.

- July 24-5, 2013: Gibraltar drops 70 concrete blocks in its waters to create an artificial reef.

- July 28, 2013: Gibraltar accuses Madrid of deliberately creating delays to car traffic with stringent checks at the border, saying vehicles are being made to wait up to six hours to cross.

- August 5, 2013: Spain's Foreign Minister Jose Manuel Garcia-Margallo threatens to impose a 50-euro ($66) toll to enter or leave Gibraltar. Gibraltar likens Spain's behaviour to that of North Korea.

- August 6, 2013: The European Commission says it will send a team of monitors to the border in September or October.

- August 7, 2013: British Prime Minister David Cameron warns Rajoy that escalating the dispute risks damaging relations between their countries.

- August 9, 2013: Rajoy vows to take "all necessary measures" to defend Spain's interests in Gibraltar.

- August 12, 2013: British warships begin setting sail for the Mediterranean in what the defence ministry stresses is a "routine" and "long-planned" exercise. One of the ships, departing Tuesday, is due to dock in Gibraltar later this week in what Spanish media have called an act of intimidation.

Britain says it is considering taking legal action against Spain over the border checks. Madrid says the checks will stay and says it is considering taking the row to the United Nations and International Court of Justice.