Britons in the path of Hurricane Irma have been warned to "have your own contingency plans in place" in case help is unable to reach them.
The Foreign and Commonwealth Office said the "situation could deteriorate significantly" and its "ability to provide assistance may be extremely limited".
The category four storm is currently battering northern Cuba and is expected to hit parts of Florida on Sunday.
It has left more than 20 people dead across the Caribbean and millions of people in Florida and Georgia have been warned to leave their homes.
Prime Minister Theresa May said work was taking place with US authorities to ensure British expats and tourists in Florida are protected.
But the latest travel advice issued by the Government says: "Across the United States, it is important that you follow the advice of the local authorities, including any evacuation orders.
"The situation could deteriorate significantly. Our ability to provide assistance may be extremely limited.
"You should ensure you have your own contingency plans in place and consider your travel plans very carefully."
Aid and expertise is being provided to Britain's territories in the region in a £32 million government cash injection.
Engineers, marines and medics are being carried on board RFA Mounts Bay, which delivered six tonnes of supplies to Anguilla and carried out repair work before moving on to the British Virgin Islands.
The Government has also pledged to double any public donations to the British Red Cross appeal for victims of Hurricane Irma, up to £3 million.
The low-lying Florida Keys will be struck first when Irma arrives on Sunday and authorities are reportedly considering withdrawing emergency teams from the islands.
The National Hurricane Centre (NHC) has forecast Irma will reach the Keys and southern Florida on Sunday morning, bringing devastating winds, rain and storm surges up to 12ft high.
- The Foreign Office has set up a hotline for people affected by the disaster and for people whose loved ones may be affected, on 020 7008 0000.