Mistakes were made in the British response to the Indian Ocean tsunami, the Foreign Office has acknowledged.
The response from the UK for Thailand was not fast enough
In the aftermath, emergency hotlines were overwhelmed, there was a shortage of medical staff and a rapid response team was sent to the wrong location.
The findings come in a joint report with the National Audit Office (NAO).
It praised staff who worked long hours under intense pressure to help Britons in the affected region last December, but said lessons must now be learned.
The Metropolitan Police's casualty bureau at Hendon ran the emergency hotline, but the 36 operators could not cope with volume of calls, which came in at 11,000-an-hour at one point.
More staff came in, but many had little or no relevant experience.
That meant mistakes were made in taking down information, which had to be checked a second time for police to carry out a missing persons inquiry.
Because initial information wrongly suggested the worst-hit area was Sri Lanka, the FCO rapid response team was sent there rather than Thailand.
As a result, reinforcements from London took two weeks to get to Thailand.
Other countries including Germany and France sent medical staff straight away, but the UK team in Thailand had only three local volunteers - a doctor, nurse and retired counsellor.
Some e-mails went unopened in the first 24 hours because of pressure on embassy staff in Bangkok.
A plea for help from Britons in Khao Lak was not seen until 29 December and assistance was "belatedly" sent.
"Foreign Office staff in the region and in London coped tremendously well under severe pressure," the report said.
"They worked extremely long hours and made great personal sacrifices in order to provide the professional and humanitarian response that British nationals expected.
"However, the traumatic circumstances and the need to make immediate vital decisions, often based on little or confused information, proved very testing.
"Mistakes were made and unintended insensitivity shown in certain cases. Officials could have benefited from more training to deal with this kind of emergency."
However, the NAO said the Foreign Office was a world leader in responding to emergencies and major improvements came after the 2002 Bali bombing.
The Zito Trust is doing a survey of Britons affected by the tsunami to find out what could be improved.
A separate report by the NAO said the rapid expansion of cheap foreign travel was putting pressure on consular services.
While overseas trips by Britons had risen during the past five years from 54m a year to 65m, the consular budget had stayed at about £80m a year.