The official toll of Britons dead or missing in the Asian tsunami disaster more than doubled today to 440, Foreign Secretary Jack Straw said.
During a visit to Thailand, Mr Straw said 49 were confirmed dead. The number of Britons unaccounted for soared to 391 from the previous official figure of 159.
The death toll now includes 36 in Thailand, 10 in Sri Lanka and three in the Maldives.
Speaking at a press conference on the holiday island of Phuket, the Foreign Secretary warned relatives that some of those unaccounted for would never be traced.
Mr Straw said relatives faced a period of "prolonged agony" as they waited for news of missing loved ones.
He said scores of experts were working to identify hundreds of bodies in one of the biggest international forensic operations ever mounted.
Mr Straw told the press conference: "Some victims may never, ever be identified and my heart goes out to all those who face this terrible and, I fear, continuing ordeal."
He said British forensic experts involved in previous incidents such as Lockerbie, the Bali bombing and the Potters Bar and Hatfield train crashes had stressed to him the size of the job.
Mr Straw said: "The agony of long uncertainty for many families and the scale of the effort still required is totally daunting.
"There are many hundreds of dead in the mortuary areas. It is impossible to tell the country of origin of most of those poor people."
At one centre, more than 500 victims of unknown nationality had arrived in the past two days, Mr Straw said.
He added: "There are teams here from 30 countries working together to identify their nationalities among the dead. This is one of the biggest international forensic operations ever mounted."
Mr Straw said the Government had taken a "careful" approach to establishing the number of British dead, relying on Metropolitan Police Service methods used in previous disasters.
His comments followed claims that the British authorities were being less open than other countries about the scale of the tragedy.
It has been suggested that police have drawn up a list of 4,000 missing Britons.
But Mr Straw said public anxiety had been greater in European countries where such "broad brush" unconfirmed figures had been released.
Mr Straw told the BBC Radio 4 Today programme there had been no delay in releasing figures to the British public.
"I gave figures on Monday and the fact that they have more than doubled in four days indicates the imprecision of estimates of this kind, particularly in the early stages after a disaster of this magnitude," he said.
"The scale and magnitude of this disaster makes it literally unique and ... bodies are still being washed up and unearthed, so the total number of casualties is still not known and as a result of that estimating the number of British people in the area and likely to have suffered is subject to a greater uncertainty than usual."
The Foreign Secretary had spent the day visiting devastated areas of Thailand, during which he met four injured British victims and relatives still searching for missing loved–ones.
He told the press conference: "Our condolences go to the loved ones of all victims – Thai, British and every other nationality.
"As I have seen here today, the very best people are working as hard as they can on behalf of the victims' relatives of every nationality."
The death toll from the tsunami disaster today stood at around 145,000 across 11 nations.
In Sri Lanka, repair work has started in earnest with the government, aid agencies and local people starting to clear up chaos caused by the tsunami.
In India, officials have said bodies were still hanging from trees and floating in waters in decimated villages on the Car Nicobar island.
Britain and the US have resisted calls to offer millions more in immediate aid to the devastated Indian Ocean region, despite UN demands for an "unprecedented global response" to the disaster.
As the total raised by the British public for the victims of the tsunami reached the £100 million mark, both countries claimed they would not be drawn into a relief "bidding war".
An emergency summit of global politicians was held in the Indonesian capital Jakarta, attended by Mr Straw, to decide how £2.2 billion of international aid already pledged could most wisely be used.
With workers on the ground warning that co–ordination of relief is failing in parts of Indonesia, the World Health Organisation said that without prompt action infectious diseases could kill as many as 150,000 people.
Britain has sent one million water purification tablets to Aceh and two RAF C–17 planes. Forklift trucks, emergency lighting and communications equipment have been provided in the region.
Further aid flights are planned to arrive in the affected countries and Royal Fleet Auxiliary Bayleaf is due to arrive off Sri Lanka to join two other Royal Navy ships, HMS Chatham and RFA Diligence, already in place.
Three chartered Mi8 helicopters are also being used by the UN in Aceh.