China must fall into line by July 31 with a World Trade Organisation order to change its restrictive policy on certain steel imports from the United States, an arbitrator said Friday.

A WTO dispute settlement hearing decided that Beijing had failed to prove why it needed more time to adapt its rules in order to respect a decision handed down by global commerce's rule-setting body in November.

In a ruling issued by the WTO, arbitrator Claus-Dieter Ehlermann said that he had decided that eight months was a reasonable period of time to take corrective action.

The dispute dates back to September 2010, when Washington accused China of breaching trade rules by not providing sufficient evidence that anti-dumping duties were needed on US imports of electrical steel used in the power sector.

Anti-dumping duties can be deployed by any WTO member if others in the 159-economy organisation sell cut-price products to grab market share.

But the rules are strict on proving that dumping has taken place, and Washington first won its case in June 2012, before a Chinese appeal failed.

Members found to be at fault are granted a period of grace to change their policy, but the defendant and plaintiff often have different views on what is reasonable, leading to yet another hearing.

The WTO has the power to authorise hefty retaliatory measures by aggrieved parties if their rivals fail to respect its rulings.