The UK is giving £15m to help survivors of the Asian sea surges, as charities step up their efforts in the region.
The first British charity aid will go to Sri Lanka and Indonesia
About 60,000 people are reported dead - many of them children - and millions homeless, after an earthquake sent massive waves across the region.
Twenty Britons have died and many more are unaccounted for, with the Thai authorities suggesting as many as 43 are known to be dead.
On Wednesday, a television appeal is expected to be made to UK audiences.
International Development Secretary Hilary Benn told the BBC the government would "make available" £15m for use in the "first phase of the relief effort", making the UK the second largest contributor after the US.
He said the effort was "about getting practical steps to get help to where it is needed" and added that the government "will do more if required".
Twelve Britons have been confirmed dead in Thailand, three in Sri Lanka and three in the Maldives, said the Foreign Office. They have not yet been officially named.
The toll of Britons will rise, with hundreds of foreign tourists still unaccounted for in resorts on the Thai coast alone.
More than £1m is already believed to have been raised by British charities, but they are aiming to generate millions more.
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
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 (email@example.com) are also raising money
Under the umbrella of the Disasters Emergency Committee (DEC), charities including ActionAid, British Red Cross, Oxfam and World Vision are asking for money to provide clean water, food and shelter to thousands.
The committee is providing 3,000 telephone lines for people to give donations - by calling 0870 60 60 900.
The government has so far donated nearly £1m and a plane-load of tents and sheeting.
Alan Duncan, shadow secretary of state for international development, welcomed the government's decision to pledge £15m to the aid effort.
He said: "We are pleased that the government raised its original derisory pledge to tackle this appalling disaster."
The Foreign Office has set up an emergency helpline - 020 7008 0000 - for people worried about missing relatives.
This 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.
Among the Britons who have been named as presumed dead are:
Meanwhile, an Oxfam charter plane carrying 27 tonnes of equipment worth £100,000 will fly to Sri Lanka and Indonesia on Wednesday.
The British Red Cross is also due to send out aid planes to the region.
Sri Lankan communities based in the UK are joining together to appeal for aid from expats, with the Sri Lankan government considering asking doctors in the UK return.
And more than 50 organisations attended a meeting arranged by the Hindu Forum of Britain in Watford on Tuesday to set up a disaster relief task force.
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.
At Gatwick, Louise Davies, 34, from Lincolnshire, told of the devastation in Galle, Sri Lanka.
"We saw local people pulling their dead loved ones from the rubble," she said. "These people desperately need help."