- Video report by ITV News correspondent Penny Marshall
The south-east coast of America was braced for impact on Friday, as Hurricane Irma moved closer to Florida's coastline after ravaging swathes of the Caribbean.
At least 22 people have died - including a two-year-old child on the island of Barbuda - and the death toll is expected to rise.
Experts said the storm could directly hit Miami as early as Friday evening, with the head of the US Federal Emergency Management Agency (Fema) warned that the storm would "devastate" the region.
"Hurricane Irma continues to be a threat that is going to devastate the United States in either Florida or some of the south-eastern states," Brock Long said.
"The entire south-eastern United States better wake up and pay attention."
Hundreds of thousands of people have been evacuated from coastal areas and told to move north, with officials warning that if they choose not to leave, they may not be rescued once the hurricane hits.
It follows catastrophic devastation reported across the Caribbean, as the storm - downgraded from a Category Five to a Category Four hurricane - tore across the island nations.
British territories were "pummelled" as Irma brought winds of up to 185mph - at its peak becoming the worst Atlantic Ocean storm in recorded history.
Officials declared a national shutdown in the Turks and Caicos islands, with life-threatening winds, rain and a storm surge expected into the weekend.
Thousands have also been left homeless by the hurricane which ripped off roofs and levelled buildings.
The prime minister of the tiny island of Barbuda said it had been left "barely habitable" with more than 800 of the 1,400 residents seeing their homes destroyed.
Its sister island of Antigua managed to escape major damage, and is now taking in those affected from Barbuda.
Resident Knacyntar Nedd said Irma's trail of destruction was "like something out of a horror movie", adding: "We had cars flying over our heads, we had 40ft containers flying left to right."
Another resident, now left homeless, said: "My whole house caved in.
"There were seven of us and all we could do pray and call for help."
As well as the toddler who died in Barbuda, one person has died in Anguilla, three in Puerto Rico, four in the US Virgin Islands and four in St Martin.
A surfer was also reported killed in Barbados.
The governor of the British Virgin Islands has declared a state of emergency.
Footage posted on social media shows deserted governmental buildings.
Thousands of British tourists believed to be holidaying in the Caribbean have been warned to follow evacuation orders.
A pregnant Briton and her sister who hadn't been heard from since Tuesday have now been found safe.
The family of Afiya Frank, who is seven months pregnant, and her sister Asha have been told the sisters were seen helping to organise relief efforts on the island.
In response to the unfolding crisis, Theresa May pledged £32 million to help the relief effort, as well as help from the Armed Forces.
Royal Fleets Auxiliary ship RFA Mounts Bay has been deployed to help deal with the aftermath of the storm with 40 Royal Marines on board, as well as army engineers and equipment.
Speaking after a meeting of the UK government's emergency Cobra committee, Mrs May confirmed that she had spoken to governors from British territories overseas and had pledged support.
The UK is working with the US authorities to ensure "everything can be done", she said.
British military support will also be provided to the French relief efforts, following a request from their government, she added.
The Royal Navy's flag ship carrier HMS Ocean has also been diverted to help, with hundreds of UK troops on board.
Meanwhile, the Queen said her and Prince Philip were "shocked and saddened by the reports of the devastation caused by Hurricane Irma".
In a statement released by Buckingham Palace, she added: "Our thoughts and prayers are with all those whose homes and livelihoods have been destroyed or adversely affected by this terrible storm.
"Please convey my gratitude and good wishes to members of the emergency services and to those who are working on the rescue effort at this very difficult time for you all."
The Red Cross estimates 1.2 million people have already been affected by Hurricane Irma.
This could rise to 49 million once Irma has passed, the United Nations said.
Even then, the misery may not be over as more storms could be on the way.
Storm Jose has been upgraded to a Category Four hurricane, which could bring winds of up to 150 mph or greater to the already-ravaged Caribbean islands.
Tropical storm warnings are in place.
Storm Katia in the Gulf of Mexico has also been upgraded to hurricane status.