Winds of up to 160mph smashed into Cuba's northern coast on Saturday hours after it was classified as a Category 5 storm.
Irma has now been downgraded to a Category 3, but is forecast to restrengthen once it moves away from Cuba,according to the US National Hurricane Centre.
Concerns are also mounting over Hurricane Jose, which is growing in strength and heading towards islands already devastated by Irma.
Florida is bracing for Irma to hit on Sunday morning. It is expected to bring massive wind and flooding damage to the fourth most populous US state.
Governor Rick Scott warned that Irma, the most powerful Atlantic Ocean hurricane in recorded history, is wider than the "entire state".
More than six million people - a quarter of the Florida's population - have been ordered to evacuate.
The Carolinas, Alabama and Georgia have also declared emergencies.
Foreign Secretary Boris Johnson said the UK was "taking swift action to respond" to the disaster after speaking to the chief minister of Anguilla, a British overseas territory that was among the first islands to be hit.
Britons in the region have been urged to follow evacuation orders, while states of emergency have been declared in Puerto Rico, Cuba and Florida - amid fears Miami could be struck directly by the hurricane.
A British naval ship has been deployed to help deal with the aftermath with 40 Royal Marines on board, as well as army engineers and equipment, as authorities struggle to bring aid to smaller islands.
President Donald Trump has cautioned people in Irma's path to "get out of its way" and not worry about possessions.
Mr Trump said property is replaceable but lives are not, and that safety must come first.
He said the nation is grieving for those who've been killed by the powerful storm, which spent the week churning its way across the Caribbean, leaving death and destruction in its wake. Hurricane Irma is forecast to hit Florida's southern coast at daybreak Sunday.
Mr Trump has again assured citizens that the US is as prepared as it can be for a storm as monstrous as Irma.
Mr Trump spoke at a weekend Cabinet meeting at the Camp David presidential retreat in Maryland.
Governor Rick Scott has told a news conference his greatest fear right now is that people will underestimate the storm surge, which is expected to be as much as 15ft in the Florida Keys.
He talks of a memory of a storm some years ago that brought six-foot storm surges into Naples, and the speed of the water crashing into homes. He says it’s hard to describe “how fast the water moved in and how fast the water moved out”.
“That was six feet,” he says. “You just think about – how can anybody survive this stuff?
Governor Scott said that more than 300 shelters are open across Florida and now is the "last chance to make a good decision" and evacuate. Or, if you are in a mandatory evacuation zone, now is the time to seek shelter on higher ground.
We are starting to get some reports from the damage Irma did to Cuba.
Irma collapsed buildings and battered Cuba with deafening winds and relentless rain Saturday, while a second hurricane, Jose, threatened to lash already-reeling islands elsewhere in the Caribbean.
Across a swath of Cuba, utility poles were toppled, trees uprooted and roads blocked. Witnesses said a provincial museum near the eye of the storm was in ruins. And authorities in the city of Santa Clara said 39 buildings collapsed.
Prime Minister William Marlin of St. Maarten - the Dutch side of St Martin - says about 1,600 tourists who were in the Dutch Caribbean territory have been evacuated and efforts are being made to move 1,200 more.
Mr Marlin says many nations and people have offered help to St. Maarten, but weather conditions will determine how this can be coordinated.
Authorities are still trying to determine the extent of damage to the island, but he said 28 police officers lost homes during Hurricanes Irma and Jose.
The prime minister said that St. Maarten remains under curfew and looting that took place immediately after the storm has subsided.
Our reporter in Hollywood, Clark Mindock, sends this:
At the Shenanigans East Side Pub in Hollywood, more than a couple dozen people packed in Saturday night to eat pizza, drink beer, and enjoy each other’s company before the worst of Hurricane Irma’s effects are felt in the area.
Outside, lightening lit up the sky, rain pelted rooftops and streets, and palm trees swayed in the increasingly powerful winds.
“We’re going to play it by ear,” one employee said while locking up TVs in a covered courtyard. “We’ll close down when the wind gets bad enough.”
Inside, people discussed whether they’d leave their homes in the morning to run errands or see friends. And, if not, what else they might do with their day.
Their phones, intermittently, buzzed with warnings of tornado watches nearby. But nobody — not the police who had stopped in at one of the last few places with an open kitchen, or the regulars who knew the bartender by name — seemed too worried about the alerts.
The US National Hurricane Center says wind gusts near hurricane strength have been recorded in the Florida Keys as the centre of Irma moves closer to the state.
Forecasters said Saturday night that Marathon had reported a wind gust of 71 mph (114 kph) and sustained winds of 51 mph (82 kph). Irma is about 105 miles (170 kilometers) southeast of Key West.
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies