A Royal Navy warship has arrived in Gibraltar the day after a stand-off between fishermen and the Royal Gibraltar Police.
HMS Westminster arrived and docked in the British territory as tensions in the region were described as the worst for 40 years.
The visit of the warship is said not to be related to the current row and is part of a long-planned deployment of a number of vessels to the Mediterranean and the Gulf.
Coming as it does at a sensitive moment for British-Spanish relations the move has been seen by some as a "strong symbol" of Britain's desire to defend its territory.
Yesterday Spanish fisherman shouting “Gibraltar is Spanish” confronted patrol boats from the Rock in protest at an artificial concrete reef put there by the government. Spanish fishermen say the reef hampers their right to fish while Gibraltar says they should not be fishing there.
Irate fishermen from neighbouring towns of Algeciras and La Linea say they have fished in waters adjacent to Gibraltar for years, and have lost earnings of €1.5m since 24 July, when the blocks were put in place.
During the protest police boats were forced to push back a flotilla of around 40 Spanish vessels.
As part of the the tit-for-tat row Spain has also introduced extra border checks causing five-hour queues for drivers trying to cross the border and has threatened to bring in a £40 charge.
The tensions between Spain, Britain and Gibraltar have led Prime Minister David Cameron to complain to his Spanish counterpart Mariano Rajoy and ask for the European Commission to intervene.
The arrival of HMS Westminster, a Type 23 frigate, which is accompanied by the Royal Fleet Auxiliary ships Lyme Bay and Mounts Bay, is likely to be a further complicating factor in the escalating row.
The group of ships, called Cougar 13, set sail from for the Mediterranean from Portsmouth and Plymouth last week. They are scheduled to dock at ports in the Mediterranean en route to the Middle East, with one warship visiting the Spanish naval base at Rota near Cadiz.
A Spanish Guardia Civil patrol boat passed close to the military area of Gibraltar harbour not long after HMS Westminster arrived this morning.
The Spanish boat passed outside the harbour walls in Gibraltar Bay before speeding off when a police launch approached it.
Julie Girling, a Conservative MEP for Gibraltar, called yesterday's flotilla a "provocative attempt to stir things up yet again" by a government in Madrid facing allegations of corruption.
"We need to find a way to make all this stop," she said.
"This has been an extended period (of border controls) and people seem to be worried that with the difficulties the Spanish Government has at home they are not going to stop, they are going to keep pushing.
"What we would like to see is the European Commission saying 'this is not proportionate'."
The floating protest was also met by Spanish Guardia Civil boats, which warned them not to sail too close to the British territory's reef.
Fabian Picardo, Gibraltar's chief minister, thanked the authorities for their help.
Mr Picardo, who has reportedly received death threats and been targeted by Spanish internet trolls, wrote on Twitter: "Big thank you also to Royal Navy, Gib Defence Police, HM Customs and Port Authority for their deployment too.
"Cool, professional and calm!"
In a statement the Gibraltar government said that its decision to create the reef had the backing of Greenpeace and Spanish environmental groups.
"HM Government of Gibraltar notes that the protest demonstrated the concerns of the Spanish fishermen of the La Linea and Algeciras 'Cofradias.' These Spanish fishermen are welcome to continue to fish in British Gibraltar Territorial Waters as long as they do so in keeping with Gibraltar law," it said.
"The British Gibraltar Territorial Waters, as defined under the United Nations Law of the Sea, are recognised by all signatories except the Kingdom of Spain."
Edward Macquisten, chief executive of the Gibraltar Chamber of Commerce, told Sky News the row has set relations between Spain and the territory back 40 years.
Additional reporting by the Press Association