Argentina has filed criminal charges against British oil firms exploring in waters near the Falkland Islands in a fresh diplomatic row over the territory, the Argentinian foreign ministry said last night.

Argentina has asked stock markets in New York and London to warn investors of its claim that five oil exploration companies are working illegally off the Falkland Islands, which Argentina contends were stolen by Britain more than a century ago.

An oil field discovery in the Falklands sparked hopes today that the area could contain as much untapped fossil fuel as the North Sea.