William Hague has hit out at Argentina after the government said it was seizing a Spanish-controlled energy firm.
The foreign secretary warned that the move by controversial president Cristina Fernandez was part of a wider protectionist agenda.
Ms Fernandez has said she will nationalise a large part of YPF, the country's biggest oil company. Spain's Repsol would see its stake reduced from 57% to 6% as a result, potentially losing billions of dollars.
The announcement - part of Argentina's efforts to curb fuel price hikes - has drawn a furious response from Madrid, which has threatened trade and diplomatic reprisals.
Mr Hague said he was "very concerned" by the developments.
"This is the latest in a series of trade and investment related actions taken by Argentina which are damaging to business interests and will undermine Argentina's economy by reducing its attractiveness to international investors," he said.
"The Argentine government has made no secret of the fact that it wishes to reduce imports and boost its domestic trade surplus through a variety of restrictive trade measures.
"This goes against all the commitments Argentina has made in the G20 to promote transparency and reduce protectionism.
"We will work with Spain and our EU partners to ensure the Argentine authorities uphold their international commitments and obligations."