The most powerful storm to threaten the US Atlantic coast in more than a decade has already left hundreds dead across the Caribbean.
Florida governor Rick Scott described it as a "monster", adding: "I'm going to pray for everybody's safety."
Flights to popular holiday destination Orlando have been affected, while the UK Foreign Office is urging people in the area to monitor weather reports and follow the advice of the local authorities.
President Barack Obama declared a state of emergency for Florida and millions of people in Florida, Georgia and South Carolina were told to evacuate their homes.
About 1.5 million people were advised to leave Florida as interstate highways were turned into one-way routes to speed up the exodus.
A Virgin Atlantic flight to Orlando from Manchester due to take off on Friday afternoon has been cancelled.
On its website, the airline is warning passengers of overnight delays affecting a number of flights on Friday and Saturday from Manchester, Glasgow and Gatwick.
The airline is telling people to check the status of their flight before going to the airport, while passengers delayed away from home are asked to remain at their hotels where possible.
Thomas Cook Airlines has delayed Glasgow and Manchester flights to and from Orlando on Friday by 24 hours due to the weather.
The airline is also telling passengers that there may be congestion at the airport in Orlando with "many flights currently being rescheduled by other airlines at this time".
In a note on its website, Orlando International Airport said flight activity was expected to stop at 8pm local time on Thursday, adding that flights were expected to resume on Saturday.
In updated travel advice, the Foreign Office said: "Hurricane Matthew is forecast to bring hazardous sea and weather conditions to parts of the east coast of the US from 6 October 2016. Strong winds, heavy rain, flooding, and storm surge in coastal areas is expected.
"Sporadic tornadoes are possible. Hurricane warnings and watches have been issued, triggering evacuation orders in several areas of Florida, Georgia and South Carolina.
"You should monitor weather reports and follow the advice of the local authorities, including if you're ordered to evacuate. For more information and advice visit the websites of the Federal Emergency Management Agency and the state emergency management authorities in: Florida, Georgia and South Carolina.
"If you're in an affected area, you should call family members and friends in the UK to let them know your plans before and after the hurricane."
The hurricane was expected to blow ashore early on Friday north of West Palm Beach, and then slowly push north for the next 12 hours along the Interstate 95 corridor, through Cape Canaveral and Jacksonville, according to the National Hurricane Centre.
Forecasters said it would then probably hug the coast of Georgia and South Carolina over the weekend before veering out to sea - possibly looping back towards Florida in the middle of next week as a tropical storm.