The treaty signed between Queen Victoria and my ancestors in 1899 covered an area of northern Canada three-and-a-half times the size of Great Britain. It guaranteed that my people "shall have right to pursue their usual vocations of hunting, trapping and fishing throughout the tract". Today, however, hunters stay away from the few moose that still roam the forests near our small community, afraid that the meat will poison their children. I remember drinking from the lake as a small child. Now, when I return to my homeland with my own young daughter, we're told not to swim because it's too toxic. This is the legacy of Canada's tar sands development in a place where my people have always lived and which is home to dozens of other First Nation communities living downstream from the sprawling tar pits. And while our people may be among the first to pay for the excesses of squeezing our earth for its last drops of oil, we won't be the last.

Global financiers betting on the tar sands are killing our last, best chance at maintaining a livable climate for everybody. As cheaper, more conventional global crude oil supplies continue to decline, an unprecedented glut of investment dollars is sloshing into the tar sands. Industry analysts expect more than $100bn (£60bn) to be invested in doubling tar sands oil production by 2020.

Fully developing these sticky oil deposits will require clear-cutting or degrading largely intact primary boreal forests covering an area the size of England. Extracting and processing just one barrel of tar sands oil requires the energy equivalent of three barrels of natural gas and two to four barrels of water, and results in a carbon footprint up to five times greater than conventional crude.

So who's behind the unprecedented expansion in the tar sands? Who are the ruthless business people throwing money at the dead-end industry poisoning our planet and trampling the rights of my community and others? Look no further than the mirror. UK taxpayers can count themselves among the biggest financial backers of the planet's most polluting industrial projects due to their majority ownership of the Royal Bank of Scotland. Since UK taxpayers bailed out RBS after it imploded one year ago, it has underwritten more than £1.6bn in debt for companies operating in the tar sands.

Today, on the anniversary of RBS becoming majority-owned by the public, 40 public figures from the UK have signed a letter to Alistair Darling, the chancellor, urging him to stop the bank from using public money to finance tar sands development, and other fossil fuel projects around the world that are having devastating impacts on the climate, local habitats and communities.

Proceeds from these deals are funding some of the most aggressive and controversial expansion projects in the tar sands. Backed by £1bn in debt underwritten by RBS this year, ConocoPhillips aims to expand production from its three tar sands projects eightfold by 2015. These are the same projects at issue in a lawsuit brought by the Beaver Lake Cree Nation seeking an injunction to end the wholesale destruction of their ancestral lands, and citing more than 17,000 infringements of the community's constitutionally protected treaty rights. RBS also underwrote $378m (£299m) in debt issued by Norway's Statoil in March of this year, just two months before the Norwegian parliament considered a motion to suspend the company's tar sands projects due to climate concerns.

As world leaders come together two weeks from now in Copenhagen, we all should take a moment to consider the way ahead. Do we go the well-worn path of big oil, ever further into the last pristine corners of the earth for our last fossil-fuel fix? Or do we forge a new path, towards a future that honours the land and the legacy of our ancestors? I hope the UK will put its money where its mouth is by pulling RBS's business out of the tar sands.