Emergency aid teams are helping evacuate people from coastal areas of Haiti in preparation for the worst of the storm.
On the US mainland, authorities fear the hurricane may slam into the Florida peninsula over the weekend, just days after Storm Harvey devastated Texas.
Forecasts suggest Irma’s most destructive winds could carve up much of Florida’s priciest real estate, damaging properties from the Florida Keys through to Jacksonville as it swirls north.
Officials are making preparations to potentially shut down two nuclear power stations in the Sunshine State, while evacuation orders have been issued in parts of Miami and the Florida Keys.
Donald Trump, whose Mar-a-Lago resort in Palm Beach could be affected by the storm, said his administration is monitoring Irma closely.
“It looks like it could be something that could be not good, believe me not good,” the US President said.
Antigua and Barbuda Prime Minister Gaston Browne told the Associated Press that Barbuda had been left “barely habitable” when the hurricane passed overhead, leaving around 60% of the island’s approximately 1,400 people homeless.
The UK Government said Irma inflicted “severe and in places critical” damage to Anguilla, which Foreign Office Minister Alan Duncan said took the full force of the hurricane. He told MPs that the
British Virgin islands have also suffered “severe damage”.
Anguilla’s tourist board said its major resorts had survived the storm, although many private homes had been damaged.
Mr Johnson said in a statement: “I spoke last night to the Chief Minister of Anguilla, and offered our condolences for the loss of life and the damage that has been sustained in his country.
“Of course we’ve been in continual contact with those in the British Virgin Islands. I want to stress that this has been a very, very severe hurricane but we are responding with all the means at our disposal.”
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 get aid to smaller islands.
French President Emmanuel Macron said he expects that victims and heavy damage will be discovered on the islands of Saint Martin and Saint Barthelemy, known as St Barts.
Briton Alex Woolfall hid in a concrete stairwell as the hurricane hit while he was on holiday in St Maarten, the Dutch area of the island.
He tweeted: “My god this noise! It’s like standing behind a jet engine! Constant booms & bangs. At least concrete stairwell not moving.”