Theresa May revealed the cash increase at Prime Minister’s Questions on Wednesday, with the new funding to come on top of the £32m already promised to help the devastated region.
She said the UK had delivered a “speedy” response to the deadly storm, despite criticism of ministers’ sluggish reaction.
The announcement came after Boris Johnson spoke of Britain’s “duty” to Caribbean islands hit by “hellish” Irma, during his visit to Anguilla and the British Virgin Islands.
Speaking to Sky News’ Foreign Affairs Editor Sam Kiley on an aid flight to the region, alongside Royal Marines and medics, Mr Johnson dismissed suggestions his visit was a PR stunt: “This has been long-planned ever since the hurricane broke.
“They’ve never seen anything like it for 150 years.
“People don’t realise that these are British people, these are British islanders and we have a duty to them.”
On his arrival in Anguilla on Tuesday, which he will tour on Wednesday before travelling to the British Virgin Islands, Mr Johnson said it was “clear this place has been through an absolutely hellish experience”.
Work is needed to fully restore power as well as the island’s hospital, airport and schools.
The Foreign Secretary’s visit was praised by Anguillan chief minister Victor Banks, who said it “sends a very positive signal to Anguillians that the British are serious about their response”.
Mr Banks revealed the UK Government had “responded within hours of the passage of the storm”, adding: “By midday the next day, the Royal Navy was here with a ship.”
But he claimed the funds being offered by the UK were “not going to be sufficient” to rebuild his island, a cost he estimated could top $1bn (£751m).
The Royal Navy’s flagship HMS Ocean left Gibraltar on Tuesday and will arrive in the Caribbean in around 12 days, laden with pickup trucks and more than 200 pallets of aid including timber, buckets, bottled water, food, baby milk, bedding and clothing.
More than 1,000 troops have been sent, with looting a key priority.
After passing through the Caribbean, Irma travelled north to Florida.
Early estimates suggest a quarter of homes in Florida Keys, where Irma first made landfall in the US, were destroyed.
President Donald Trump is due to visit Florida on Thursday.
King Willem-Alexander of the Netherlands and French President Emmanuel Macron have both visited their countries’ respective Caribbean territories.
At least 37 people were killed in the Caribbean, with 12 dead in Florida, four killed in South Carolina and two in Georgia.