The number of Britons killed or likely to have perished in the Asian tsunami has risen to 453.
UK soldiers have been clearing up debris in Sri Lanka and elsewhere
Of those, 51 are now confirmed dead, said Prime Minister Tony Blair in a statement to the Commons on Monday.
He defended the efforts of the Foreign Office, which has been criticised by some grieving families, before pledging a further £25m in immediate aid.
A memorial service for the tsunami victims, attended by the Queen, would be held later this year, said Mr Blair.
Arrangements for the service will be made after consultation with the relatives of those who died.
By the end of last week 50 Britons were confirmed dead and 391 missing, feared dead. Most were holidaying in Thailand when the waves struck.
The total death toll across the affected region stands at more than 150,000.
Twenty inquests have so far been opened and adjourned at Westminster Coroner's Court in London, into the deaths of those whose bodies have been flown home.
There was "total unity" in the House of Commons in sending condolences to all victims of the disaster on 26 December, Mr Blair said.
He said 871 Britons were still unaccounted for in the region, down from more than 2,000 last week, but were not thought "highly likely" to have been victims. This figure may never fall to zero, he said.
The amount of immediate aid from the UK government would rise from £50m to £75m, none of which would be taken from existing development programmes, he said.
He told MPs that cash committed in various aid packages, tax relief on charity donations and suspension of debt repayments from afflicted countries came to about £200m.
Scotland Yard has also set up a specialist unit of intelligence officers to advise charities on the possible threat of sex abuse to child victims of the tsunami.
The National Child Protection Authority in Sri Lanka has welcomed the move.
There have been allegations that criminal gangs are befriending children orphaned by the tsunami and selling them to sex traffickers.
Conservative leader Michael Howard said the UN should produce regular independent audits of the money pledged and received, while the government should publish figures for the amount pledged and spent by the UK.
The leaders of all three main political parties again paid tribute to the generosity of the British public, who have so far donated £100m to the Disasters Emergency Committee's tsunami appeal.
In response to criticism of UK officials Mr Blair said that under the difficult circumstances in which the Foreign Office, police and other staff were operating it was inevitable there would be occasional mistakes made and "unintended insensitivity in certain cases".
"The complexity of the operation is obvious. The grief of the families is manifest and absolutely understandable. The circumstances couldn't be more taxing," he said.
KEY UK SUPPORT
£75m for immediate humanitarian relief
Foreign Office temporary offices set up in affected areas
Assistance with cost of repatriation of remains, medical expenses, travel for relatives
More than 200 family liaison officers helping victims' families
75 police officers sent to Thailand and Sri Lanka
Support for UK-based families who have lost relatives abroad, especially Sri Lanka
Early emergency airlifts to Maldives and Indonesia
Equipment for UN field office in Banda Aceh
Five helicopters plus vehicle for UN use in Aceh province
Five flights to Indonesia to deliver 4,000 family tents
Two helicopters from Brunei-based Gurkha battalion to Indonesia
RAF C17 plane, five C130 planes and two naval vessels deployed to help with aid distribution
But staff had done a "magnificent and exceptional job".
The Foreign Office has been criticised by some relatives of those missing as being slow to offer support, while many are frustrated at the slow process of identifying bodies.
Many victims were swept away by the force of the waves and their remains may never be recovered, said Mr Blair.
In some cases DNA testing would be used, which could be a slow process.
"I know this will only add to the agony of the families concerned. I'm afraid no short cuts, however, are possible.
"The pain and upset which could be caused by mistakes would be even worse."
He said British officials and police were now working with the families of victims to repatriate remains, where appropriate, and helping the injured get home.
Mr Blair defended the decision not to immediately release the potential number of UK victims, saying that figure - which would have been about 7,000 - could have caused "unnecessary upset and alarm for people".
Urgent work was needed on an early-warning system, he added.
And he had asked the government's chief scientific adviser, Sir David King, to put together a small group of experts to advise on the detection and early warning of natural hazards.