Canada, seeking to close one of the darkest chapters in its history, will formally apologize on Wednesday for forcing 150,000 aboriginal children into grim residential schools, where many say they were abused.

Prime Minister Stephen Harper will rise in parliament at 3 p.m. (8 p.m. British time) and read a half-hour apology for what happened at the church-run schools, which mainly operated from the 1870s to the 1970s.

"There are thousands of hearts and minds that will be in different stages of acceptance, but I hope that we will begin the process of healing and reconciliation," Harper told Parliament on Tuesday.

Survivors will be in parliament for the occasion. The schools were initially set up to educate native children but later became part of a government campaign to assimilate aboriginals and eradicate their culture.

Contemporary accounts suggest up to half the children in some institutions died of tuberculosis and other diseases.

Many survivors say they were abused mentally, physically and sexually. Children were beaten for speaking their own languages and told they would be damned unless they converted to Christianity.

Native leaders say the damage done at the schools is directly responsible for the many social problems that today afflict the country's 1 million aboriginals.

In May 2006 Canada reached a C$1.9 billion (969 million pounds) settlement with the roughly 90,000 school survivors, who say an apology is crucial to help them overcome their trauma.

"I am one of many thousands that have lived this tragic experience and I, as many many others, have been very troubled by this experience," said Phil Fontaine, who heads the Association of First Nations.

"I personally have come a point in my life where this apology will enable me to put this behind me in a very real way. it's a very important moment," he told the Canadian Broadcasting Corp. on Wednesday.

A formal truth and reconciliation commission started work on June 1 and will spent the next five years travelling the country to hear from school survivors.

The scandal is reminiscent of what happened during the same period in Australia, where at least 100,000 aboriginal children were removed from their parents. Prime Minister Kevin Rudd apologized to the "Stolen Generations" in February.

The atmosphere ahead of the Canadian apology has been tense, with aboriginal groups complaining they were not consulted about the wording.

Opposition legislators say native representatives should be allowed to respond in parliament, an idea Harper dismissed.

"Aboriginal Canadians have been waiting for a very long time to hear an apology from the Parliament of Canada. I would urge all parties not to play politics with this," Harper said on Tuesday.