Spain's police have been criticised for a stunt in which divers filmed themselves inspecting a concrete reef created by Gibraltar in disputed waters
The divers took flags with them and posed underwater for photos before posting them on social networking site Twitter.
"The act of diving itself constituted a serious violation of British sovereignty but this apparent interference with the reef is a new and worrying aspect," said Sir Adrian Johns, the Gibraltar Governor, speaking to the Daily Telegraph.
The police's actions were a "blatant attempt" to exercise jurisdiction in waters over which Gibraltar claims sovereignty, he added.
Divers from the Guardia Civil can be seen in a video measuring and inspecting one of 74 concrete blocks placed in the water by Gibraltarian authorities last month to create a reef, a move which has been met with anger by Spain.
The country also claims sovereignty over the waters around the self-governing territory, which measures just 6.8 square km (2.6 square miles) and is home to just under 30,000 people.
The Gibraltar government released a statement on the incident, saying, "Her Majesty's Government of Gibraltar notes the incident of executive action taken by the Guardia Civil in British Gibraltar Territorial Waters in the area of the new artificial reef.
"The matter of this serious incursion will not assist in de-escalating the present tensions."
Sir Adrian commented that the police dive was "particularly unhelpful" in the light of what he described as Gibraltar's "conciliatory position" in a heated row over sovereignty and fishing rights in the small British-held territory.
Fabian Picardo, the territory' chief minister announced on Saturday that 59 Spanish fishing boats could return to the disputed waters within months, under new legislation, in an attempt to calm tensions with the Spanish. He insisted, however. that the controversial reef would remain.
Gibraltar currently bans fishing with nets under an environmental protection law passed in 1991, but which Spanish fisherman were exempted from until Mr Picardo's Socialist Labour government came to power in 2011.
The Spanish government is demanding that the Gibraltarians dismantle the reef, which it says prevents its fishermen from operating in waters over which the country also claims ownership. Madrid has also accused its neighbour of deliberately damaging the Spanish fishing trade.
The flag stunt is that latest in a series of moves that have ratcheted up tensions between the two governments.
The European Commission will send observers to the border between Gibraltar and Spain at the request of both sides after the Spanish imposed extra border checks on those wishing to enter the territory.
The UK says these actions, which have caused lengthy traffic delays, break rules on free movement within the European Union. Spain's explanation for the checks were that the British outpost has not done enough to prevent smuggling.