Around 440 Britons are either dead or missing in the Asian tsunami disaster, Foreign Secretary Jack Straw has said.
Mr Straw said the effort still required in Asia was "truly daunting".
Mr Straw told a news conference in Thailand 49 Britons had been confirmed dead plus 391 unaccounted for. One more death was announced later.
The confirmed dead included 37 Britons in Thailand, 10 in Sri Lanka and three in the Maldives.
Back in Britain Cherie Blair said the public's donations had been
"fantastic", while Princes William and Harry have carried out volunteer work.
In Thailand the foreign secretary said the anxiety of relatives of the missing would be prolonged because of the scale of the task to identify bodies.
Mr Straw said experts from more than 30 countries were involved in one of the biggest international forensic operations ever mounted to identify victims.
"The agony of long uncertainty for many families and the scale of the effort still required is truly daunting," Mr Straw said.
British forensic experts involved in previous incidents such as Lockerbie and the Bali bombing had stressed the size of the job, he said.
"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."
Meanwhile Cherie Blair - the prime minister's wife - praised the British public's response to the disaster during a visit to a Save the Children charity shop on Friday.
Speaking at the shop in Clapham, south London, where she donated bags full of toys, she said: "The response has been fantastic. I would urge the public to continue to donate."
It also emerged on Friday that Princes William and Harry have been working as volunteers in a Gloucestershire warehouse sending aid to tsunami victims.
Mr Straw, who has been meeting the families of British victims on the island of Phuket, added that many unidentified victims were still being found.
"In the past two days more than 500 victims of currently unknown nationality have arrived for examination, " he said.
The foreign secretary said as well as the Britons confirmed dead, 391 people arenow listed by the Metropolitan Police as "likely or very likely to have been involved as victims."
"That is, for example, that there is an eyewitness account of them in thewater or a damaged building; an eyewitness knows that they were in the immediatearea and they have not been seen since", he said.
If it is confirmed the Asian tsunami has claimed the lives of 440 Britons, it will be the highest British death toll since the end of World War II.
Officials in Thailand said China had offered the use of its laboratories for DNA testing and the first samples from the remains of unidentified victims would be sent there later on Friday.
US Secretary of State Colin Powell and UN Secretary General Kofi Annan are travelling to Sri Lanka to witness the impact of the disaster there.
In Brussels, European foreign ministers are to meet to discuss how the £1bn in aid pledged by the EU should be spent.
Cherie Blair praised the public response while visiting a charity shop
The meeting, which also includes EU aid and health ministers, will focus on planning long-term reconstruction for the affected countries.
Most of the emergency aid provided so far by the European Union and European Commission has been spent on food and water, clothing and shelter to survivors.
More than 140,000 people are now known to have died in the Asian tsunami disaster, and hundreds of thousands more are homeless.