The British Foreign Office says Spanish vessels have repeatedly entered their waters around Gibraltar, describing the incursions as a "clear violation of international law."
Gibraltar's government says Spain's drugs and money laundering police squad have been escorted from the waters after following suspects in the area.
In one incident the Spanish are thought to have been pursuing smugglers who were dumping suspected bales of drugs off the side of a speeding inflatable boat.
"These repeated incursions into British Gibraltar territorial waters are a clear violation of UK sovereignty by another EU country and we will be raising this as a matter of urgency with the Spanish authorities," Hugo Swire, a Foreign Office minister, said in a statement.
