The European Commission said Tuesday that it is acting at Spain's request to investigate the use of concrete blocks to create an artificial reef off Gibraltar, a British crown colony at the tip of the Iberian peninsula.

"As soon as we receive a formal complaint from a member state, we are obliged to launch a complaint procedure, which could in turn lead to an infringement procedure," EC spokesman Olivier Bailly said in Brussels.

The dropping of 70 concrete blocks into the Mediterranean violates the European Union's environmental regulations and threatens the livelihoods of Spanish fishermen, Madrid says.

The EC will initiate an infringement procedure over the reef only if "we were able to confirm the facts and were not convinced by the answers Gibraltar and the United Kingdom provide," Bailly said.

"Obviously, we're not there yet," the EC spokesman said.

The probe of the reef will be carried out independently of the investigation prompted by complaints about hours-long waits at the Spain-Gibraltar border since Madrid imposed new checks, Bailly said.

EC President Jose Manuel Durao Barroso and Spanish Prime Minister Mariano Rajoy discussed both aspects of the Gibraltar situation in a telephone call on Monday.

The two men "agreed that a Commission fact finding mission should as soon as possible examine in loco the border control/movement of people and goods questions," the EC said in a statement.

Spain insists the new border checks are legal and necessary because neither Britain nor Gibraltar is part of the Schengen Area, which allows passport-free travel across borders, and because London chose to exclude the Rock from the European Customs Union.

Britain must remove the concrete blocks from the water as a precondition for dialogue between Madrid and London, Spanish Foreign Minister Jose Manuel Garcia-Margallo said in an op-ed published Tuesday in The Wall Street Journal.

Some 6,700 Gibraltarians live in Spain but do not list it as their tax residence.

Gibraltar, a territory of 5.5 sq. kilometers (2.1 sq. miles) at the entrance to the Mediterranean Sea, has been held by Britain since 1704 and became a British Crown Colony in 1713 under the Treaty of Utrecht. EFE