In a speech in Japan this week, Professor Hata traced the start of the fighting to an ill-timed toilet stop by a Japanese soldier. He said reports that the soldier was missing surfaced not long after a dozen shots were fired at Japanese troops at the western end of the bridge on the night of July 6.

The Japanese army telegraphed the Chinese forces the next morning, saying a soldier was missing. They believed he had been kidnapped and taken to the town of Wanping to the east, and demanded entry to search for him.

The request was refused by the Chinese army holding the bridge, and that night Japanese artillery began bombarding the town. The Chinese fought back and the conflict escalated into a war that would last eight years and claim more than 35 million lives.

Professor He insisted the missing solider and the gunfire claims were fabricated by the Japanese army, which needed a pretext for the invasion of the mainland.

He says that according to a Japanese commander's witness account, the soldier was found 20 minutes after he was reported missing, and the Chinese army said no shots were fired.

Japan illegally sent troops into China and established the puppet state of Manchukuo in northeastern China in March 1932, and from that point intended to control the entire northern part of China, according to Professor He.

He said the war would have broken out at some point that year because the Japanese army, encouraged by its easy success in Manchuria, was seeking an excuse to launch further aggression.

But in his book The Marco Polo Incident, 1937, Professor Hata cast doubt on the argument that the Japanese were ready for an all-out war. They had fewer than 6,000 soldiers in China at the time, while the Nationalist 29th Army holding Beijing and several nearby regiments commanded nearly 200,000 infantry soldiers, he argued.

His opinion represents the mainstream view in Japan that it was an accident and the Chinese were more responsible for its escalation.

The 2005 edition of a Japanese history textbook says: 'The incident itself was just a small conflict.

'Although people hoped that it could be peacefully resolved, conflict continued to occur with the Japanese side, and it became difficult to find a solution.'

This was the position reflected in a series published in Japan's biggest newspaper, the Yomiuri, which aimed to clarify responsibility for the war. In the series, the incident was described as 'a brief unplanned battle between the Imperial Japanese Army and Chiang Kai-shek's Chinese National Revolutionary Army'.

Professor He said the Japanese always tried to blame China for the war.

'The answer to the question directly affects our understanding of the nature of the war,' he said. 'Any attempt try to whitewash the invasion must be seriously contested.'