The Union of British Columbia Indian Chiefs says it is "angered, disgusted and deeply disturbed" by the distribution of a book promoting residential school denialism by the wife of a mayor in the Canadian province.

The First Nations representative organisation raised concerns over Pat Morton, the wife of the mayor of Quesnel, Ron Paull, circulating the book Grave Error – How the Media Misled Us, which claims genocide did not occur at Residential Schools, and "questions the very existence of Residential Schools and their well-documented harms against Indigenous peoples".

The Union (UBCIC) expressed its solidarity with the Lhtako Dene Nation, who formally sent a letter to the Quesnel Mayor and Council expressing their outrage and asking that Mayor and Council reaffirm the terms of the Memorandum of Understanding between the City and themselves.

The Lhtako Dene Nation also requested that the Mayor and Council meet with some of their Elders, who were forced to attend Residential School, to hear first-hand accounts of the notorious residential school system.

Canada's Truth and Reconciliation Commission of Canada investigated the period 1883 to 1996 and eight years of research determined that more than 150,000 First Nations, Métis and Inuit children were sent to residential schools after being removed from their families.

Isolated from their families and prohibited from observing their culture and using their language, many of the children were physically and sexually abused, and vast numbers died of disease, malnutrition or neglect.

More than 1,300 mass graves have been discovered on the grounds of abandoned residential schools in recent years, leading to apologies from Pope Francis.

UBCIC President Grand Chief Stewart Phillip said the Union "categorically rejects the denialism and ardent dissemination of misinformation put forward by the authors of this book (Grave Error – How the Media Misled Us), and perpetuated by members of the public and Pat Morton".

"We are concerned that this is just the tip of the iceberg and could represent a broader network of individuals connected to municipalities peddling hateful denialism and eroding trust in local government," he said on Tuesday.

"Holocaust denialism has been widely documented and debunked as the work of people who feel threatened and choose to perpetuate long-standing stereotypes and hateful beliefs used to undermine the realities of history."

Grand Chief Phillip said the UBCIC calls for all levels of government, and the public, to uphold the testimony of those with lived experience who survived and witnessed horrors at Residential Schools, the findings of experts, and the documentation of physical evidence captured in the Truth and Reconciliation Commission's Final Report and by subsequent investigations by First Nations and partners at former Residential School sites across Canada.

UBCIC Secretary Treasurer, Chief Marilyn Slett, noted that Canada's policy of forced assimilation of Indigenous children at Residential Schools was intentional and is widely documented.

"The pain of this denialism is deeply offensive and compounds the suffering that generations of survivors have already endured," she said.

"UBCIC holds up survivors and their families who lived through atrocities, as well as the children who never made it home, and all those who are today harmed by the actions of those involved with the distribution of this book and related materials. We stand with you in the truth, we believe you, and we demand accountability for the crimes of Residential Schools which will not be shrouded off by the ignorance of a hateful few."

Chief Slett said Lhtako Dene Nation has the "full and unwavering support" of the UBCIC in calling on the Mayor to reaffirm their Memorandum of Understanding and to meet with their Elders who survived Residential Schools.

UBCIC Vice-President Chief Don Tom urged Mayor Paull to condemn Residential School denialism, and said the Union calls for the mayor and Ms Morton to undergo cultural sensitivity training, to publicly apologise to Lhtako Dene Nation and to all survivors of Residential School, and to implement the Truth and Reconciliation Commission's Calls to Action and advance municipal policies that "acknowledge the history and harmful legacy of Residential Schools".

"Other mayors and political leaders who are responsible for championing reconciliation efforts, take this as a reminder that the company you keep matters; we will not permit the distortion of fact, hateful denialism in or near our political institutions, or the further harm of our people," Chief Tom said.

The UBCIC noted that the Quesnel Mayor and Council was set to hear from Grave Error contributor Frances Widdowson during their next council meeting and expressed shock at the decision.

The Union noted that Ms Widdowson was fired from Mount Royal University based on her "atrocious comments on Canada's residential school system and the Black Lives Matter movement" and that she "continues to question the findings bravely shared by Tk'emlúps te Secwepemc and other First Nations about unmarked graves as being profit motivated".

The Union of British Columbia Indian Chiefs slammed the Mayor and Council for "wasting time by providing Ms Widdowson space on a taxpayer-funded agenda, rather than focusing on proven facts and working with First Nations toward reconciliation".

Last January, after a compensation settlement for victims of the residential school system was announced, Shane Gottfriedson of Tk'emlups Indian Band - a former British Columbia chief of the Assembly of First Nations - said the system "decimated our languages, profoundly damaged our cultures, and left a legacy of social harms".

"The effects go beyond my generation. It will take many generations for us to heal," he said.