Gordon Brown is set to apologise for the UK's role in sending more than 130,000 children to former colonies where many suffered abuse.
The Child Migrants Programme, which ran from the 1920s to the 1960s, sent poor children to a "better life" in Australia, Canada and other elsewhere
But many were abused and ended up in institutions or as labourers on farms.
The prime minister is due to express regret for the scheme in the Commons in a move first announced in November.
More than 130,000 children, aged between three and 14, were sent to Commonwealth countries.
As they were shipped out of Britain, many were wrongly told their parents were dead, while parents were given very little information about where their children were going.
'Important milestone'
The former child migrants have told how on arrival they were separated from their siblings, and put into foster homes, state-run orphanages and religious institutions. Others were educated only for farm work.
Many were subjected to brutal physical and sexual abuse by those who were meant to be caring for them.
HISTORY OF UK CHILD MIGRANTS- UK the only country with a sustained history of child migration - over four centuries
- In 1618, 100 sent from London to Richmond, Virginia
- In total 130,000 sent from the UK to Canada, New Zealand, South Africa, Zimbabwe (formerly Rhodesia) and Australia
- Post-war, 7,000 shipped to Australia and 1,300 to New Zealand, Rhodesia and Canada
Ordeal of Australia's child migrants
Mr Brown announced he was planning to apologise in November, when Australia's Prime Minister Kevin Rudd said sorry for his country's part in the tragedy.
No British prime minister has ever delivered an official apology, despite repeated demands from victims' groups.
Last week, Britain's High Commissioner to Australia, Baroness Amos, said the apology would be an "important milestone".
"Over the past few months I have met many whose lives were blighted, and heard their personal stories," she said.
"We want not just to bear witness to the past but to look forward to a future where these terrible events will not be repeated."
Harold Haig, secretary of the International Association of Former Child Migrants and their Families, said: "For many former child migrants and their families, the apology will help to heal a painful past."
The wording of the apology by Mr Brown is believed to have been discussed with charities representing former child migrants and their families.
Sixty survivors have apparently been flown to London so they can listen to the statement in person. Some 7,000 child migrants from Britain still live in Australia.
The prime minister is also expected to make an announcement about future support for those affected.
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