The British Virgin Islands and the British overseas territory of Anguilla have suffered "severe" damage from Hurricane Irma.
Updating the Commons, Foreign Office minister Sir Alan Duncan said Anguilla had taken the full force of the storm, causing "critical" damage in some places.
At least one person has died on Anguilla, the Caribbean Disaster Management Agency said, adding that 90% of the island's roads were impassible.
The territory's airport and hospital have also suffered damage, as well as its power lines and phone networks.
Some residents on the affected Caribbean islands have criticised the British government for not doing enough to help.
But Sir Alan told MPs that a Royal Navy ship will reach affected islands on Thursday with tents, vehicles and other relief equipment.
"We really have complete overall concern particularly for our overseas territories which are affected and to that end we have £12m immediately available through our rapid response mechanism for disaster relief and recovery," Sir Alan told MPs.
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The damage is "not as severe as first thought" on another British territory, Monsterrat.
A further UK territory, the Turks and Caicos Islands, is expected to be hit by the storm later on Thursday.
Travel association ABTA said thousands of British tourists are believed to be holidaying in the Caribbean.
Sir Richard Branson is counting the cost of widespread damage at his private retreat in the British Virgin Islands.
"Glad to say that all humans on Necker are ok although a lot of buildings destroyed," his son Sam Branson said on social media.
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France's interior minister, Gerard Collomb, said eight people had died on the French side of St Martin, adding that the figure was likely to rise in the coming hours.
The Dutch Royal Navy said "enormous damage" had been caused by Hurricane Irma on the Dutch side of the Caribbean island.
Many houses are missing roofs and hotels are underwater, it added.
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Dutch Prime Minister Mark Rutte said the storm had been of "epic proportions", causing "widescale destruction of infrastructure, houses and businesses".
"There is no power, no gasoline, no running water," he said.
"Houses are under water, cars are floating through the streets, inhabitants are sitting in the dark in ruined houses and are cut off from the outside world."
There was "no clarity" on victims.
Chair of a local council on St Martin, Daniel Gibbs, told Radio Caribbean International it was an "enormous disaster, 95% of the island is destroyed".
"I am in shock," he said.
St Martin and Anguilla experienced heavy rain and winds of up to 185mph, while damage in St Barts was described as "apocalyptic" after winds of 151mph.
A two-year-old was killed on Barbuda as a family tried to escape a damaged home during the storm.
Antigua and Barbuda Prime Minister Gaston Browne told Sky News the "carnage in Barbuda is unprecedented".
About half of the island's 1,800 residents have been left homeless.
Nine in 10 of its buildings have been damaged - some of them completely destroyed.
Mr Browne said gusts of winds had reached 230mph, and 40ft containers had been carried 100 yards from their original location.
In contrast, the damage in Antigua had been "nominal, to the extent that it is business as usual today".
Irma was over Puerto Rico earlier on Thursday, where it left more than 900,000 people without power and nearly 50,000 without water.
An economic crisis in Puerto Rico has resulted in major funding and staffing cuts to the public power company.
The power supply may not be restored for up to six months as a result.
As many as 49 million people could be affected by Hurricane Irma, the United Nations has warned.
The category five hurricane - the most powerful Atlantic storm ever recorded - is moving on a course towards Haiti, Cuba and the Bahamas, before being expected in the US state of Florida by Sunday.
America's National Hurricane Centre said it was moving off the northern coast of the Dominican Republic on Thursday with maximum sustained winds of 180mph, and was likely to drop down to category four as it reached Florida.
The threat Florida faced was continuing to increase, it added.
The head of America's Federal Emergency Management Agency, Brock Long, told CNN that Irma would have a "truly devastating" impact when it hits the US.
In Florida, roads were packed with vehicles and there were petrol shortages, prompting governor Rick Scott to tell people to only "take what they need".
Mr Scott has described the storm as "life threatening", telling people to follow evacuation orders because "you can rebuild your home - you cannot rebuild your life".
President Trump tweeted: "Hurricane Irma is raging but we have great teams of talented and brave people already in place and ready to help. Be careful, be safe!"