Having battered the north coast of Cuba as a category three storm, deadly gusts approaching 130mph swept across the Florida Keys.
Parts of Miami were later under water as more than one million homes were without power on Sunday despite the storm being downgraded again to category three.
The hurricane hit the Sunshine State with its forward reaches sending “embedded tornadoes” sweeping across southern parts.
Irma’s eyewall - a band of clouds surrounding the centre of the storm - reached the low-lying Keys island chain on Sunday morning, with the eye of the storm 15 miles southeast of Key West.
More than six million people in Florida and Georgia have been urged to leave their homes, while tens of thousands of people were huddling in shelters as the National Hurricane Centre warned the storm would bring 130 mph winds, torrential rain and storm surges of up to 15ft.
Florida governor Rick Scott called on anyone still in at-risk areas to follow evacuation orders, saying: “This is clearly a life-threatening situation.”
“If you have been ordered to evacuate you need to leave now. This is your last chance to make a good decision,” he said on Saturday night. “Evacuation procedures are in place across the state, more than 6.5 million Floridians have been ordered to evacuate. Do not put yourself or your family’s life at risk.”
Monitoring the situation from Camp David, President Donald Trump urged people in the danger areas to heed the governor’s advice.
Foreign Secretary, Boris Johnson, said there were more than 125 British troops on the British Virgin Islands working with the local police, while more than 50 British police were on their way, after the storm had devastated the Caribbean.
The presence of the troops has had “a massive psychological effect” on morale on the islands, said Mr Johnson after an emergency Cobra committee meeting on Sunday.
Three planes were on their way to deliver crucial supplies to the region, he said, while the UK will be sending military operation to Anguilla, one of the first islands to be hit by Irma and receive aid, as soon as possible to ensure it “does not now slip behind”.
The Royal Navy ship RFA Mounts Bay, carrying engineers, marines and medics, delivered six tonnes of supplies to the British overseas territory and carried out repair work before moving on to the British Virgin Islands.
Responding to suggestions the UK response was not good or quick enough, Mr Johnson said: “Well I would say that’s completely wrong. RFA Mounts Bay was in the region for a specific need and....that is exactly the right type of boat to have in the region.
“Other countries actually now been asking us for help including the French, we’ve got three planes going out today, it’s an unprecedented effort by the UK to meet what has been an unprecedented catastrophe in that part of the Caribbean.
“But I’ve absolutely no doubt that we can face up to the challenge, we can deliver the help that those islanders need, we are seeing some signs of improvement, things are getting better on BVI, we’ve now got to make sure Anguilla gets the help it needs.”