The trial will be open for the hearing of the bribery charges but will be closed to deal with charges of stealing commercial secrets.

Speaking to reporters in Canberra on Thursday, Kevin Rudd, the Australian prime minister, said his government would do all it could to support the interests of the Rio employees.

"The world is watching how this particular court case is conducted," he said.

Trial access

Stephen Smith, the Australian foreign minister said diplomats should have full access to the trial under a consular agreement with Beijing.

The case threatens to strain diplomatic and trade ties between the two nations [Reuters]

"I was disappointed that there was an indication from Chinese officials and the court that Australian officials would not be present, or be able to be present, for the commercial information charge," he told reporters.

Under the commercial secrets charge, courts can jail people for up to three years, or up to seven years in serious cases. The bribery charge could draw jail terms of up to 20 years.

Rio Tinto has said the four did nothing wrong. If convicted, the men can appeal.

It is unclear how long the trial will last, but Chinese trials are usually short, with a wait of days or weeks before a verdict is announced at a separate hearing.

China's courts come under the heavy influence of the ruling Communist Party and rarely reject prosecution arguments.

Iron ore negotiations

The case comes amid already strained relations between Australia and China and it threatens to further cast a pall over trade ties.

China is Australia's second-largest export customer behind Japan, and more than half of China's imports from Australia last year were of iron ore.

Rio Tinto, based in London and Australia, is the world's third-largest mining company and one of China's biggest suppliers of Iron ore, one of the main ingredients for making steel.

In June last year, it ditched a planned $19.5bn investment by state-owned Chinese metals firm Chinalco, and there was speculation in Australia that the arrests may have been in reprisal for the collapsed deal

The deal with Chinalco would have been one of the largest ever foreign investments by a Chinese company.

But Rio Tinto executives changed their mind and instead launched a tie-up with fellow Anglo-Australian miner BHP Billiton amid concern that the deal would have given China too much control over Australia's natural resources.