Argentina warned British firm Premier Oil PLC, which wants to team up with Rockhopper to seek oil near the disputed Falkland islands, that such activities would be illegal.
"Oil and gas exploration by the firm Rockhopper Exploration PLC on the Argentine continental shelf are illegal and as such the Argentine republic has declared the company to be operating illegally ... and has taken appropriate legal measures," a foreign ministry statement said in a letter sent to Premier through the Argentine embassy in London.
Premier Oil on July 13 announced it had taken a 60-percent stake in Rockhopper operations north of the South Atlantic islands, held by Britain but claimed by Argentina since 1833.
Potential oil wealth is a key factor in the South Atlantic territorial seas and subseas dispute, one of the few such disagreements in the region still fuelling tensions.
In June the countries marked the 30th anniversary of the end of the Falklands War. Argentina's invasion of the South Atlantic islands in 1982 triggered a 74-day war that left 255 British soldiers and 650 Argentines dead.