Florida governor Rick Scott tweeted during the early hours of Wednesday morning: “This is your last chance to evacuate before conditions start deteriorating within the next few hours.”

The storm has been tracking across the Gulf of Mexico over the past few days before forecasts of it making landfall on the US mainland where hundreds of thousands of people have been advised or ordered to evacuate.

More than 20 million people in the states of Florida, Alabama, Georgia and North and South Carolina were placed under a hurricane or tropical storm warning by Tuesday evening, the Guardian reports.

A state of emergency has been declared in roughly half of Florida’s 67 counties. Wakulla county sheriff’s office warned in a Facebook post: “This storm has the potential to be a historic storm, please take heed.”

Michael is expected to bring heavy rain to the Gulf Coast, a four-metre storm surge and winds of up to 120mph.

The US National Weather Service said: “Hurricane Michael is forecast to make landfall in the FL Panhandle on Wednesday. Life-threatening storm surge expected along the Panhandle, Big Bend and Nature Coast.

“Very dangerous winds are expected along the coast, and hurricane-force winds will be felt far inland, including parts of GA and AL. Heavy rainfall is likely to cause inland flash flooding. Tornadoes are also possible.”