A total of 26 Britons have been confirmed dead after the Indian Ocean tsunami, with Thai authorities suggesting as many as 43 may have died.
Rescue workers remove tourists' bodies from a Thai beach resort
About 77,000 people are reported killed - many of them children - and millions are homeless, after an earthquake sent massive waves across the region.
The UK Government is sending £15m in aid, while British charities have raised £5m in the last 24 hours.
Three Britons died in the Maldives, three in Sri Lanka and 20 in Thailand.
This number is expected to rise, with thousands of foreign tourists still unaccounted for in the affected areas.
International Development Secretary Hilary Benn told the BBC the government's aid donation would be used in the "first phase of the relief effort".
HOW TO DONATE
The Disasters Emergency Committee (DEC) is an umbrella group of UK charities including, among others, British Red Cross, Cafod, Oxfam, Save the Children, Tearfund
Call them on 0870 60 60 900 or donate online at www.dec.org.uk
Other bodies raising money include Muslim groups Muslim Aid (020 7377 4200) and Islamic Relief (0121 622 0622)
Sri Lankan organisations including Asia Quake Relief Appeal UK (firstname.lastname@example.org) are also raising money
Prime Minister Tony Blair said the world was "united in sorrow" and that Britain would help those affected "in any way we can".
"We are constantly increasing our aid and support and are determined to ensure that everything we provide is directed to where it is needed and can best be used," he said.
The Disasters Emergency Committee (DEC), an umbrella group of British charities said it had received almost 140,000 phone donations, while more than £300,000 had been pledged online.
Brendan Gormley, DEC chief executive said the response had been "phenomenal". He said he hoped the total would double overnight as television and radio advertisements were broadcast.
British digger manufacturer JCB has donated US$1m (about £522,000) worth of equipment to assist the clean-up operation.
The DEC is providing 3,000 telephone lines for people to give donations - by calling 0870 60 60 900.
The Foreign Office has set up an emergency helpline - 020 7008 0000 - for people worried about missing relatives.
But the line has come under fire, with relatives saying they have been unable to get through, or have been put on hold for minutes on end and then cut off.
An extra 2,200 lines were installed on Tuesday.
Foreign Secretary Jack Straw defended the helpline when he visited its call centre in north London.
Louise Willgrass, from Norwich, was killed on Phuket
"When people do get through they can have the reassurance their call will be taken by a trained police officer or trained civilian member of staff," he said.
Among the Britons who have been named as presumed dead are:
0207 008 0000 - for information on friends and relatives
0870 6060290 - for flight details or travel advice
British survivors returning from the region to the UK on flights to London and Manchester have been describing their experiences.
Mick Byrne, 42, from Brighton, had been staying in Phuket with his wife and daughter.
"There were bodies floating in and out of the sea. It was absolutely terrible," he said at Heathrow Airport.
Nick Ward told how he and a friend helped rescue up to 50 people from the surging water on Phi Phi island in Thailand.
"It's just what anybody would do. I just did as much as I could."
A spokeswoman for the Association of British Travel Agents (Abta), Frances Tuke, told the BBC News Website travel operators were working feverishly to ascertain the exact number of British tourists killed in various resorts across Asia.
The first British charity aid will go to Sri Lanka and Indonesia
She said up to 100 were unaccounted for in Sri Lanka but added: "They may be travelling back on a scheduled flight having abandoned their holiday."
She said every Briton in the Maldives had been accounted for, although there were several fatalities there.
Ms Tuke said those on package holidays had access to help but many people, especially in Thailand, were travelling independently without any support network.
She urged them to contact their families at home to let them know they were safe.