Lawyers said that Google and the US might have a legal basis for suing China at the World Trade Organisation, a move that would further strain trade relations between Washington and Beijing.

Over the past year, each government has brought a string of emergency trade restrictions and legal cases against the other. Last month, the WTO appeals body handed the US a victory, upholding an earlier ruling that China had broken WTO rules by requiring foreign companies to use Chinese distributors for music, books and films, including products delivered online.

The US could argue that Beijing's censorship in effect discriminated against foreign services such as Google, contrary to its commitments under the General Agreement on Trade in Services (Gats).

“If China imposes harsher web filtering restrictions on Google than on local search engines, such as Baidu, Google may have a WTO discrimination claim,” said David Spooner, a former assistant secretary of commerce in the George W.Bush administration, now at the law firm Squire Sanders & Dempsey.

The outcome of a case would depend on how a WTO dispute resolution panel classified search engines. Much of the WTO law addressing internet services and online products is unclear. The last global trade agreement was negotiated in the early 1990s when the technology was in its infancy. But trade experts said a succession of rulings had narrowed governments' room for manoeuvre, and particularly their ability to use national security or the protection of public morals as defence for censoring words and images on the web.

Gary Horlick, a leading international trade lawyer, said: “We will have to know a lot more about the facts, especially what the [Chinese] government is doing, but the Gats has a lot of unex­plored obligations which might protect Google.”

The remaining question is whether the US government would want to bring yet another case against Beijing. The US trade representative's office was unavailable for comment. But Hosuk Lee-Makiyama, a fellow at the European Centre for International Political Economy, a think-tank, said: “The Obama administration has made a priority out of enforcing existing trade agreements, and [US trade representative] Ron Kirk has emphasised the importance of the digital economy and protectingint­el­lectual property rights.”