Thailand's worst floods in half a century reached the edge of downtown Bangkok on Friday, threatening some underground rail stations and forcing the closure of a major shopping centre.

About 20 percent of the capital is now submerged in floodwater contaminated by rubbish, dead animals and industrial waste, raising fears about outbreaks of disease in the densely populated metropolis of 12 million people.

The slow-moving water is now just a few kilometres (miles) away from business and tourist districts, despite reassurances from the government that central Bangkok would be spared.

The floodwater arrived at the Lat Phrao intersection on the northern edge of the city centre early Friday, prompting the closure of the Central Plaza shopping mall.

A spokesman for the Bangkok metro said that three subway stations -- Lat Phrao, Phahon Yothin and Chatuchak Park -- were at risk and might have to be shut down if the water rose to 40 centimetres (16 inches) outside.

"We have deployed officials at all risk stations to assess the situation hourly," he said.

Nearby Chatuchak Weekend Market -- a popular tourist attraction -- will re-open this weekend at the request of traders, but officials have warned them to be on alert for possible flooding.

The floods -- caused by unusually heavy rains and failure to release enough water from dams in the early part of the monsoon -- have killed 442 people and damaged the homes and livelihoods of millions around the country.

The authorities have issued an evacuation order for eight Bangkok districts out of a total of 50 in the capital, and for certain areas in four others.

The 12 districts have a combined official population of 1.7 million people -- far more than government shelters can accommodate.

Many have chosen to stay in their homes despite risks including electrocution, disease and lack of food and drinking water, complicating relief efforts.

The authorities are attempting to drain the floods through waterways in the east and west of the sprawling capital and out to sea. But many roads have also become rivers as the seemingly unstoppable mass of water creeps southwards.

Worst-hit residents have complained that their homes are being sacrificed to save downtown Bangkok's shopping malls, luxury hotels and the houses of the wealthy elite, triggering protests and the destruction of some flood barriers.

The waters have swamped Bangkok's number two airport Don Mueang, which usually serves mostly domestic destinations, but the kingdom's main air gateway Suvarnabhumi east of the city centre is operating as usual.

The main airport is surrounded by a 3.5 metre high flood embankment monitored around the clock, officials told media invited to see the defences.

Suvarnabhumi general manager Somchai Sawasdeepon said the airport would put its emergency plan into effect if the water reached 2.5 metres deep.

"I believe we can evacuate all passengers and aircraft within 24 hours," he said. "Now the airport is operating normally. There are no signs that worry me. The water level also doesn't concern me."

Thai authorities failed to save a number of major industrial parks from the floods, despite earlier assurances they would be protected.

The crisis is taking its toll on the lucrative Thai tourism industry, with countries including the United States, Britain, Singapore, Canada and the Netherlands advising against all but essential travel to Bangkok.