Up to nine thousand Britons are thought still stranded in Mexico's Yucatan peninsula, up to five days after seeking shelter from Hurricane Wilma.
Wilma has destroyed thousands of buildings in Mexico
A Foreign Office rapid deployment team and the UK's Mexico ambassador, Giles Paxman, have travelled to the region.
Mr Paxman expressed concerns about conditions in some storm shelters.
Wilma later swept over southern Florida, where there are about 30,000 UK tourists and 70,000 expats, the Foreign Office said.
The Foreign Office (FO) estimated on Tuesday that there were between 8,000 and 9,000 British tourists in the affected region of Mexico.
It flew a team of eight consular officials, a British Red Cross counsellor and International SOS doctor to the area on Monday.
This follows the deployment there of a 15-strong consular team, including the ambassador, from across the region.
Mr Paxman told the BBC: "Thankfully there have been very few injuries and no [British] fatalities, but people have been living in shelters or on hotel floors now, some of them for three, four or even five days, and the conditions are not, obviously not, conditions of great comfort.
"There have been some shortages of food and water, some people are on medication and are worried that their medication is going to run out, and sanitation is in some cases very poor."
Terry Preece, of Cardiff, told BBC Radio Five Live on Tuesday that he was among a group of holidaymakers now sleeping in the street in Cancun, Mexico.
"There's 56 of us stuck out in a street in downtown Cancun, women and children up on the roof, elderly people, and we're out in the street basically, and we're all really stressed out," he said.
Police in Cancun have fired shots to try to control hundreds of looters in the aftermath of the hurricane.
Association of British Travel Agents (Abta) spokesman Sean Tipton said flights in and out of Cancun had been cancelled "at least until Wednesday".
The FO said its emergency phone line set up on Sunday had taken "many" calls from Britons worried about relatives caught up in Hurricane Wilma.
The hurricane left Mexico on Sunday after pounding the peninsula, killing at least six people and destroying hotels and thousands of homes.
It later swept over Florida, killing at least four people and leaving some three million homes and businesses without electricity.
Mr Tipton said people due to travel to affected parts of Florida had been offered alternative destinations or refunds.
Package holidaymakers already there were being looked after by holiday reps and had shelter, he said.
"We're taking steps now to ensure they can be flown home at the earliest available opportunity."
Consular teams from Atlanta, Georgia, and Orlando were stationed in the affected areas of Florida, an FO spokesman said.
Another rapid deployment team from London was also on stand-by, he added.
Jack Anderson, who moved to Naples, Florida, from Fife, in Scotland, told BBC News: "There's quite a bit of damage to some of the houses round about, and I know that some of the manufactured homes have been flattened in some areas."
British holidaymakers in Cuba have been waiting anxiously after flights from Havana to the UK were cancelled due to intense rain and high winds.
The Foreign Office helpline number is 0207 008 0000.