Brussels has three other US cases pending

The issue is a US accounting practice known as "zeroing", which the European Commission says results in financial penalties for EU firms.

Zeroing is a method of calculating penalties for dumping goods on a market at below cost-price - and in this case, Brussels argues, often makes EU exporters unfairly liable.

This is the fourth transatlantic WTO case that the Commission has launched recently, adding to disputes over steel, genetically-modified foods and US export credits.

Level playing field

The problem with US zeroing rules, the Commission says, is that they diverge from international practice.

"The EU has already abandoned this practice and has several times asked the US to do likewise, in order to ensure a level playing field," said European trade commissioner Pascal Lamy.

"Unfortunately, the US refuses to move on this issue, and regrettably we are left with no choice but to refer the matter to the WTO."

Brussels wants the WTO to set up formal consultations under its dispute-settlement mechanism.

Steel and spaghetti

According to the Commission, trade worth "several hundreds of millions of dollars" is involved.

Zeroing mainly affects EU exports of steel, chemicals and pasta.

The WTO has ruled against zeroing once before, in a complaint brought by India against the EU.