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.

The FTSE 100 Index slumped more than 2% today after markets across the globe were shocked by a downgrade of the US government’s debt outlook.

The FTSE 100 Index slipped further below the 6000 mark today as heavily-weighted banking stocks suffered from renewed European debt concerns.

Tensions between the UK and Argentina mounted today as a British oil rig began drilling off the Falkland Islands.