UK charities have already raised an "unprecedented" £25m to ease the suffering of Asian tsunami victims.
Water shortages could cause many more deaths
The Disasters Emergency Committee, an umbrella group for leading charities, said it had been "overwhelmed" by the response to TV and radio appeals.
With the death toll currently at 114,000, 29 Britons are confirmed dead and many more are missing.
The UK government is now the largest international donor after increasing its £15m donation to £50m.
'Scale of destruction'
International Development Secretary Hilary Benn said: "We have looked at the initial assessments from our teams on the ground.
"It is now sadly clear that the scale of destruction and loss of life is increasing all the time.
"I have said from the start of this crisis that we would make available the money needed."
There are fears that the death toll will soar because of disease and water shortages.
More than £5m had already been raised by the public before Wednesday night's TV and radio appeals which led to further "overwhelming generosity" .
The DEC website, where online donations can be made, has struggled to cope with "a high volume of traffic" with some visitors urged to try again later or make a telephone donation.
Hundreds of thousands of telephone pledges were made following Wednesday night's appeal to take the total to an "unprecedented" £20m by Thursday morning.
DEC spokeswoman Martha Clark said: "We have been inundated with calls - but if people are patient they will get through."
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
By the end of the week people should be able to donate cash or cheques - made payable to the DEC Tsunami Earthquake Appeal - at High Street banks.
Cash or cheques (made payable to Post Office Ltd) can be donated over the counter at Post Office branches.
Other bodies raising money include the Muslim groups Muslim Aid (020 7377 4200) and Islamic Relief (0121 622 0622) and the Hindu charities Sewa International 0116 261 0303 and the ISKCON Disaster Appeal on 01923 856848.
Sri Lankan organisations including Asia Quake Relief Appeal UK (email@example.com) are also raising money
But organisers have urged people to continue giving because of the scale of the disaster.
On Thursday, police forensic experts were travelling to Asia to help identify victims.
And a Red Cross plane containing 40 tonnes of logistics and telecommunications equipment left Bristol Airport to Colombo, Sri Lanka, at 1620 GMT.
The equipment will be used by a team of logistics experts who flew to Colombo from London on Monday to make sure Red Cross supplies are distributed efficiently.
Prime Minister Tony Blair has pledged that Britain is ready to help those hit "in any way we can".
But the government has had to defend itself against accusations that it is not offering enough money. It has since increased its donation.
A Buckingham Palace spokeswoman said the Queen would be donating a "substantial amount" to agencies of which she is patron.
Details of the agencies would be released later, she added.
Meanwhile, the BBC has learnt that British officials in Phuket, Thailand, are finalising an airlift of about 350 UK citizens "too injured to travel comfortably" on commercial planes.
BBC correspondent Chris Hogg said the plane to London would also be used to bring back some of the bodies of tourists killed by the floods.
The recovery operation in Phuket was now about searching for bodies and not survivors which was why the death toll was rising so rapidly, he said.
The Thai government was trying to source refrigerated lorries to preserve bodies for as long as possible so they could still be identified.
But some tourists were now swimming and sunbathing on Patong beach, he added.
British ambassador in Thailand, David Fall, said many hundreds of Britons may still be missing.
"We are talking, at the least, in the twenties of deaths, injured in the hundreds - we know that because people have been coming through hospitals," he told BBC Radio 4's Today programme.
There could have been thousands of British people in Phuket and other affected areas at the time of the disaster, he added.
Other British aid efforts include a donation by digger firm JCB of US$1m (about £522,000) worth of equipment to assist the clean-up operation, and a party organised by fans following the England cricket team in South Africa.
Mobile phone firm Vodafone has also donated £1m to the relief effort.
The Federation of Tour Operators, which has been co-ordinating the flying of empty
planes to the region to bring holidaymakers home, will now organise the transport of humanitarian aid on the outward flights.
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.