More than a million people today joined a mass exodus from Houston and southern Texas as Hurricane Rita intensified into one of the most powerful storms on record and threatened widespread devastation to the Gulf Coast.

Evacuees fleeing the region sat in bumper-to-bumper traffic jams along all of the major interstate highways as families sought shelter inland.

The 300-mile wide hurricane, which has tonight been downgraded from Category 5 to 4, became the third strongest storm ever recorded this morning with winds gusting to 210mph and an atmospheric pressure of just 897 millibars.

Forecasters predict that Hurricane Rita will weaken slightly before hitting Texas on Saturday but expect it to arrive as no less than a Category 3 storm with winds of up to 130mph.

The path of the storm shifted slightly to the east this afternoon and is now expected to make landfall somewhere between the city of Galveston, which sits on a low-lying sand barrier, and Port Arthur, on the border of Texas and Louisiana. At 1600 BST, the eye of the storm was 460 miles off shore in the Gulf of Mexico.

"It’s not a good picture for us at this point," said Steve LeBlanc, the city manager of Galveston. "We’re in for a historic storm."

Weather forecasters have told Galveston officials to expect the storm surges of Hurricane Rita to overwhelm the 17-foot (5-metre) seawall that protects the city. The wall was built after Galveston was destroyed by a hurricane in 1900 that killed 8,000 people.

Once making landfall, Hurricane Rita is expected to drive north and lash the city of Houston, which lies 50 miles inland, and bring 18in rain and flooding to the region, which is the centre of the American oil industry.

Officials have warned that just three inches of rain could overwhelm nearby New Orleans, where protective levees are still not fully repaired from Hurricane Katrina. The city is so fragile that even a glancing blow could bring fresh devastation.

In Washington, President Bush met with emergency management officials and said that government agencies at all levels were preparing for disaster.

"This is a big storm. And it’s really important for our citizens there on the Texas coast to follow the instructions of the local authorities," said Mr Bush.

"Officials at every level of government are preparing for the worst. Our armed forces have pre-positioned troops. We have resources there to help the federal, state and local officials to respond swiftly and effectively."

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