The British government on Tuesday summoned Spain's ambassador in London to receive a protest over an incursion by a Spanish research vessel in what Gibraltar claims as its territorial waters.

Federico Trillo was told that London considers such excursions to be both unlawful and "unacceptable," Britain's Foreign and Commonwealth Office said in a statement.

This is the third time in the last 12 months that FCO has called in the Spanish envoy in connection with Gibraltar, a territory of 5.5 sq. kilometers (2.1 sq. miles) at the entrance to the Mediterranean Sea that has been held by Britain since 1704 and became a British Crown Colony in 1713 under the Treaty of Utrecht.

The latest incident involves the Ramon Margalef, a Spanish oceanographic survey ship that entered Gibraltarian waters on Monday and remained there for more than 20 hours, despite the British Royal Navy's repeated requests for the boat to withdraw, the FCO said.

Three Spanish Guardia Civil patrol vessels were escorting the Margalef on Tuesday as boats from Gibraltar lingered nearby.

The skipper of the Margalef responded to the requests to withdraw by insisting that the vessel was operating with the permission of Spanish authorities for the benefit of the European Union, according to the FCO statement.

"I strongly condemn this provocative incursion and urge the Spanish government to ensure that it is not repeated," Britain's minister for Europe, David Lidington, said Tuesday.

"We stand ready to do whatever is required to protect Gibraltar's sovereignty, economy and security," he said.

Spain's foreign minister said Madrid was surprised by Britain's protest over the Margalef.

The survey ship is collecting data for the European Commission's probe into a Gibraltarian project to create an artificial reef, Jose Manuel Garcia-Margallo said from Miami, where he was accompanying Crown Prince Felipe and Princess Letizia on an official visit to the United States.

"Spain is acting within the most strict respect for international legality," the foreign minister said.

The Gibraltarians claim jurisdiction over the waters within three nautical miles of the Rock, but Spain says that under the Treaty of Utrecht, only the port waters belong to Gibraltar.

Madrid want Gibraltar reincorporated into Spain, an outcome Gibraltarians have rejected overwhelmingly in referendums, while Britain maintains that the views of the Rock's residents must be taken into account.

Tension over Gibraltar flared in July when the local administration on the Rock dropped 70 concrete blocks into the Mediterranean with the aim of forming an artificial reef.

The project violates EU environmental regulations and threatens the livelihoods of Spanish fishermen, according to Madrid, which imposed stepped-up border checks that have led to hours-long waits for people entering and leaving Gibraltar.

Britain complained about the border checks to the European Commission, which said last Friday that Spain's additional custom inspections at its border with Gibraltar do not violated EU law. EFE