The central bank has shifted the central peg of its dollar band twice this week in a calculated move that suggests Beijing aims to offset the precipitous slide in Chinese manufacturing by trying to gain further export share abroad.

The futures markets are pricing in a 6pc devaluation over the next year. "This is clearly a big shift in policy and we are now on alert," said Simon Derrick, currency chief at the Bank of New York Mellon.

The move follows a Politburo speech by President Hu Jintao warning that China is "losing competitive edge in the world market".

China has allowed a crawling 20pc revaluation over the past three years. Any reversal risks setting off conflict with the incoming team of President-Elect Barack Obama in Washington. Mr Obama called China a "currency manipulator" during the campaign, a term that carries penalties under US trade law.

Outgoing US Treasury Secretary Hank Paulson is viewed as a "friend of China". He called for a stronger yuan this week before embarking on a visit to Beijing, but the plea was couched in friendly terms. This soft-peddling may soon change.

Hans Redeker, currency head at BNP Paribas, said China's policy switch could set off a dangerous chain of events. "If they play this beggar-thy-neighbour game, it will cause a deflationary shock for the whole world," he said.

It makes sense for countries with current account deficits such as the UK, US or Turkey to let their currencies fall, but China has the world's biggest trade surplus.

Michael Pettis, a professor at Beijing University, said it was "very worrying" that a pro-devalulation bloc seemed to be gaining the upper hand in the Communist Party. "I really do believe that we are on the brink of a very ugly period for trade relations," he said.

China has relied on exports to North America and Europe as its growth engine, making it acutely vulnerable to the contraction in global demand. Mr Pettis said this recalls the role played by the US in the 1920s, a parallel fraught with danger. "In the 1930s the US foolishly tried to dump capacity abroad, but the furious reaction of trading partners caused the strategy to misfire. China already seems to be in the process of engineering its own Smoot-Hawley," he said, referring to the infamous US Tariff Act in 1930.

China showed restraint during the Asian crisis in 1998, holding the line against domino devaluations across the region. It may yet hold the line this time.

However, this crisis is more serious. The manufacturing sector has seen the steepest decline since the records began, with devastation sweeping the textile, furniture and toy sectors. Civil unrest has begun to rock the Guangdong and Longnan regions.

Beijing has slashed rates and unveiled a fiscal stimulus of 14pc of GDP, but most of the spending comes in the form of instructions to local governments to spend more – but without giving them the money. Does China really intend to step in to prop up global demand? The jury is out.