It is China's worst flooding for more than 40 years.

Millions have been left homeless

Officials said the precautionary evacuations, in Gongan county in the central province of Hubei, were because a fourth flood peak, caused by further torrential rain, was rushing downstream.

Authorities in the city of Wuhan, in Hubei, have reportedly started moving around 300,000 people in an attempt to save the city.

An official told the BBC that most of the population was being relocated from Gongan county, an area designated as a flood diversion basin.

The Xinhua news agency said people had taken whatever possessions they could carry and joined the exodus to higher land.

The area is the cornerstone of a flood diversion project put to the test only once before, in 1954, when floodgates were opened to divert surging waters. It is hoped this will again protect the large industrial city from the rising waters.

A sign of the threat posed by the river came when part of the main dyke was breached close to the downstream city of Jiujiang, in Jiangxi province.

Many are homeless in the Jiangzhou township

The Xinhua news agency said soldiers were trying to block a 40 metre gap by sinking boats and building a secondary dyke using tonnes of stones.

It said shops had been closed and residents urged to leave the area, though officials contacted by the BBC said the city centre had not been affected.

Some settlements built inside the river's main dykes are reported to have been flooded already in an attempt to reduce pressure on the banks.

The China Daily newspaper quoted the chief engineer at the local hydrological bureau as saying the area was facing the worst challenge in its history.

Millions hit

Chinese officials say 240 million people in 28 provinces have been affected by the flooding.

Mud and landslides have been responsible for most of the deaths.

Officials say 13.8 million people had been evacuated, 5.5 million houses destroyed and 4.78m hectares (11.7m acres) of crops ruined.

The government has pledged 0.5kg (1.1b) of grain every day for each of the flood victims and said Beijing had provided more than 1.9bn yuan ($229m) in relief funds on top of 720m ($99m) from provincial governments.