A UK military relief flight is being sent to the Caribbean after the British Virgin Islands declared a state of emergency following Hurricane Irma.
The islands' governor has asked the UK for help, amid reports of widespread devastation, with fatalities reported.
One resident said the hurricane, which hit on Wednesday, was "terrifying" and had left "total devastation".
The UK has almost tripled the relief fund for British overseas territories to £32m amid criticism of its response.
Irma passed over the British overseas territories of Anguilla, Montserrat and the British Virgin Islands (BVI) on Wednesday night - and went on to batter the UK islands of Turks and Caicos further north on Thursday evening.
The BVI is a collection of 40 islands and islets with a population of more than 30,000 - Tortola is one of the largest, home to more than three quarters of the population.
Briton Emily Killhoury lives on Tortola with her husband Michael and their two children, aged nine and 10. She told the BBC her family bunkered down in a closet when the storm hit.
"Our downstairs doors suddenly blew out, which was terrifying. We just stayed hiding," she said.
"We eventually emerged at about 7pm to see total devastation. Everybody is shocked, but trying to be practical."
Sir Richard Branson, who refused to leave his private retreat of Necker Island in the British Virgin Islands (BVI), said his island and the whole area has been "completely and utterly devastated".
In a blog, Sir Richard said his team were safe and well after taking shelter in his concrete wine cellar, but he added: "I've never seen anything like this hurricane".
"We are still assessing the damage, but whole houses and trees have disappeared.
"Outside of the bunker, bathroom and bedroom doors and windows have flown 40 feet away.
"I'm speaking these words from a satellite phone that is just about working, but all other communications are down."
British overseas territories are self-governing but rely on the UK for protection from natural disasters.
Former UN head of humanitarian relief Baroness Amos said on Thursday that it was felt the UK "did not respond" quickly enough to the disaster.
But Prime Minister Theresa May insisted that the government had responded "swiftly".
What has happened in British territories?
- Anguilla British Overseas Territory hit by the full blast of the hurricane on Wednesday. At least one death reported.
- British Virgin Islands Reports of casualties and fatalities and extensive damage. Expected to require extensive humanitarian assistance. In a message the people of the BVI, governor Gus Jaspert said: "I come to you with a heavy heart after experiencing and observing the extent of devastation caused by Hurricane Irma." Communications are difficult.
- Montserrat British overseas territory "swiped" by Irma but suffered less serious damage.
- Turks and Caicos Low-lying British Overseas Territory battered by the hurricane on Thursday night, with roofs ripped off, streets flooded, utility poles snapped and a widespread black-out on the main island of Grand Turk.
What is the UK doing?
Humanitarian workers with 200 shelter kits and the Royal Navy ship, RFA Mounts Bay, were sent to the area before the hurricane struck.
The military vessel arrived in Anguilla on Thursday, with personnel there clearing roads and helping to restore power.
HMS Ocean is also being sent from the Mediterranean but is not expected to arrive for another two weeks.
The first British military flight to join the relief effort will leave RAF Brize Norton later, carrying around 200 troops - including engineers, marines and medics, as well as rations and water. Other transport aircraft are expected to follow with helicopters on board.
At the scene: 'Foolish to rush in'
By BBC South of England correspondent Duncan Kennedy, RAF Brize Norton
Loading aid is a complicated operation and the first of the RAF flights to leave Brize Norton will take off this lunchtime.
It will have around 200 Royal Marines onboard, together with water supplies and shelter equipment.
The RAF base commander here has denied the British have been slow to get going. He says it would be foolish to rush into the region before knowing exactly what is needed.
There will be more flights from Brize later, with a total of around 300 military personnel heading to the Caribbean.
What is the advice for travellers?
Holiday firms are monitoring the situation and some have cancelled flights or offered to amend bookings for those due to travel to affected areas in the coming days.
The cruise company Carnival has cancelled four cruises bound for the Caribbean that were due to depart over the next few days - and warned that others may be cut short.
The Foreign Office urges people planning to go to the areas to follow the advice from the local authorities, including any evacuation orders, and check its official travel guidance before travelling.
It has set up a hotline for people affected by the disaster and for people whose loved ones may be affected on 020 7008 0000.