A total of 131 Britons are unaccounted for after Hurricane Katrina hit the US, the government has said.
Many of those would most likely be safe, but efforts were underway to try to locate them, Foreign Office minister Lord Triesman said.
Following criticism of help given to UK survivors by British officials, he said everything possible was being done.
The first Britons rescued from New Orleans have arrived back in the UK, with more expected on Monday.
They were among thousands finally being evacuated from the city, after days in appalling conditions in rescue centres.
Lord Triesman said the Foreign Office had received phone calls from people who were worried about relatives they believe are in area struck by the hurricane.
Lord Triesman said the government was trying to help affected Britons
He said Foreign Office staff were not being allowed into the affected areas.
"We required the agreement of the Louisiana governor to get into the city, and we haven't been given that agreement," he said.
"From the first hour we provided financial support for hotels for clothing, for medical aid ... we've made arrangements with airlines for people to fly back and assisted with medical requirements," he said.
Lord Triesman said he thought the consular staff who had co-ordinated aid in Houston, Dallas and San Antonio in Texas had done "extraordinarily well" to cope with the problems.
He also urged those people who had managed to make contact with relatives or friends to contact the Foreign Office, so people could be taken off the missing list.
An earlier estimate put the number of Britons unaccounted for at up to 150, but the Foreign Office later said the correct number was 131.
A spokeswoman said officials were keen to stress that many of those were likely to be safe and could have fled the disaster days ago.
"As in the aftermath of the tsunami disaster, we are likely calls about many more people than the number who may actually be missing," she said.
About 20 Britons evacuated from the city's Superdome have already flown back to the UK, while others await flights.
Will Nelson, 21, arrived at Gatwick Airport on Sunday morning after spending five days in the "pretty intimidating" Superdome.
He said: "Everyone was pushing and shoving and shouting. It escalated into chaos really. Everyone was getting more desperate. Everyone was running out of food."
Adam Friend, 21, of Exeter, told BBC News he felt unsafe, saying: "It turned into a race issue in there...it was only a minority of the people that did it, but they made us feel very unsafe, throwing bottles at us.
Criticising the UK embassy response, Mr Friend said: "I have never known a more useless bunch of people in my life. The press managed to gain access to the Superdome but they couldn't."
Wayne Henry 48, who flew to America to find his son, Peter, was also critical.
He said: "I have no confidence in the British government. They are not doing enough for people out here," he told BBC News.
Sir Nicholas Young, chief executive of the British Red Cross, urged Britons to give money to help those affected by the disaster.
"Lives have been shattered and livelihoods lost. We are doing all that we can to help our sister organisation the American Red Cross meet people's immediate needs."
The Ministry of Defence is sending 500,000 ration packs - armoured metal boxes which contain enough food for 24 hours - which will be flown to the US on Monday morning.
Relatives of Britons in the areas when Katrina struck can call a UK helpline: 0207 008 0000 or contact the British consulate in Houston on 001 713 659 6270.
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