The official toll of Britons dead or missing in the Asian tsunami disaster has more than doubled to 440.

And experts are working through a list of some 2,000 others reported missing after the Boxing Day tragedy whose cases are of "much, much less concern", a Metropolitan Police source said.

During a visit to Thailand, Foreign Secretary Jack Straw said 49 Britons were confirmed dead while 391 were "likely or very likely" to have been involved as victims.

The confirmed death toll includes 36 in Thailand, 10 in Sri Lanka and three in the Maldives.

If all those in the official estimate are confirmed to have died, the tsunami will have claimed more British lives than any other single event since the Second World War.

"Overall, this therefore gives an estimated total of 440 British casualties for the region as a whole, the vast majority, tragically, in Thailand," Mr Straw told a press conference on the holiday island of Phuket.

He said some missing victims would never be traced. Relatives faced a period of prolonged agony as they waited for news of missing loved ones, he warned.

Mr Straw said scores of experts were working to identify hundreds of bodies in one of the world's biggest forensic operations.

He added: "Some victims may never, ever be identified and my heart goes out to all those who face this terrible and, I fear, continuing ordeal."

The global death toll from the tsunami disaster now stands at around 145,000.

US Secretary of State Colin Powell has visited Sri Lanka to inspect the relief work being carried out in the devastated island.

He arrived in the capital, Colombo before being flown by helicopter to Galle in the south of the country.

The British public has pledged some £100 million for nations affected by the disaster.

Mr Straw 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 task.

"The agony of long uncertainty for many families and the scale of the effort still required is totally daunting," 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."

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."

The Foreign Secretary had spent the day visiting devastated areas of Thailand, during which he met injured Britons and relatives still searching for missing loved-ones.

He 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.

Mr Straw said public anxiety had been greater in European countries where "broad brush" unconfirmed figures of the dead and missing had been released.

The category for those "likely or very likely" to have been involved in the disaster includes cases where an eyewitness saw the victim in the water or a damaged building, or where an eyewitness knew the victim was in the immediate disaster area but has not heard from them since.

This category also includes Britons known to have been in the affected region on Boxing Day who have not made contact since then, even though they were expected to do so.

Mr Straw 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."

So far the bodies of 19 Britons have been returned to the UK. Inquests will be carried out at West London Coroner's Court.

A north London call centre set up to deal with inquiries about the tsunami had received 135,000 calls as of yesterday.

The Metropolitan Police are appealing for relatives to contact the call centre again once they have received news of loved ones previously feared missing.

Workers on the ground have warned that co-ordination of relief is failing in parts of Indonesia.

And in India, officials have said bodies are still hanging from trees and floating in waters in decimated villages on the Car Nicobar island.

The European Union was today due to start mapping out its strategy of longer term economic and financial assistance for the tsunami-hit nations.