The total aid pledged to relieve the victims of the Asian tsunamis has now reached nearly four billion dollars, the United Nations revealed tonight.

The UN's emergency relief co-ordinator said that the world was working together in an unprecedented fashion to help the 150,000 people killed and five million left homeless by the Boxing Day disaster.

As Britain came to a standstill for three minutes' silence at noon today to pay respects to the dead and bereaved, nations in the developed world appeared to compete to ever-increasing amounts of money to the relief cause.

John Howard, the Australian Prime MInister, said that his country was raised its donation to $750 million, while Germany increased its pledge to $668 million. The European Union is due to agree a further $132 million in addition to the $30 million it has already promised.

Jan Egeland, the UN’s emergency relief coordinator said that the generosity was "just incredible".

"We are recording now pledges between three and four billion dollars," he said. "It is indeed the world coming together in a manner we’ve never, ever seen before."

Relief agencies who are traditionally bitter competitors for limited quantities of aid money say that for once rivalries have been set aside, and they are managing to work together to ensure the record offers of aid reach the victims quickly.

"There is an unprecedented openness and spirit of cooperation, unlike on some other occasions where some agencies felt the need to make their individual presence felt," said Jamie McGoldrick of the UN Office for the Coordination of Humanitarian Affairs (OCHA) in Geneva.

In Britain, flags on government buildings and at Buckingham Palace fluttered at half mast as the nation paid its respects. 200 British tourists are among the dead or missing, making it this country's worst natural disaster of recent times.

In town centres, shops and offices across the UK, millions paused to join the silent tribute, which was observed across most EU states an hour earlier. The gesture was suggested last week by Luxembourg, which holds the rotating EU presidency.

Television channels aired silent footage of the aftermath of the tragedy, as the start and end of the silence was announced in the country's biggest shopping centres, in mainline railway stations, on trains and at airports.

But the first national silence observed since the September 2001 attacks has been criticised in some quarters as 'grief inflation' and berated as an empty gesture. Roger Gale, MP for Thanet and a Conservative Party vice-chairman, said: "I believe that this is the wrong initiative at the wrong time. Some self-styled 'world leaders' - including the Prime Minister - have shown very little leadership when it has been needed in recent days.

"While the public have leaped forward to help, governments have lagged behind. Those present in this country at the weekend are well aware that people of all denominations and faiths gathered in places of worship to mourn their dead and to pray for the missing and the injured.

"They do not need a state-imposed three-minute pause to underline their feelings at this stage."

Mr Blair has been criticised for deciding to continue with his family holiday in Egypt after the disaster and the Government came under fire for its initally hesitant approach to the aid effort. Questions have grown why the Government's aid pledge of £50 million has been so far outstripped by private donations from the British public, expected to reach £100 million by the weekend.

This morning the Prime Minister defended his decision to remain on holiday. He told BBC Radio 4's Today programme: "The important thing is, it's action not words. The British people have responded with tremendous generosity...Throughout the entirety of the time, I have been intimately involved with all the decisions that have been taken and I'm doing now pretty much what I was doing then."

He said he expected that the Government's final commitment to aid and reconstruction projects in southern Asia would reach several hundred million pounds, far outstripping individual donations.

Jack Straw, the Foreign Secretary, left last night for Jakarta, the Indonesian capital, where he will meet heads of government tonight before representing the G8 at a tsunami summit tomorrow. The conference will discuss setting up a tsunami early warning system for the region, and is expected to debate how the money pledged so far will be spent.

In addition, Kofi Annan, the UN Secretary General, is expected to use the conference to launch an appeal for a further $950 million in international aid for the UN to channel to the tsunami-affected countries over the next six months.

Although the global total pledged by donor countries is nearly $4 billion, far less than that has actually been promised to the UN itself, reflecting mistrust of the body in some quarters. Mr Annan has also expressed concern that not all the money will be forthcoming.

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