REGINA -- Saskatchewan Premier Brad Wall says any attempt by Ottawa to link the money it transfers to a province with that province's carbon tax policy would be a serious attack on federal-provincial relations.

Wall says in a letter to Prime Minister Justin Trudeau that such action violates the principles of fiscal federalism and calls the threat unacceptable.

Trudeau has said all provinces must set up a cap-and-trade system or impose a price on carbon of at least $10 per tonne starting next year -- increasing to $50 by 2022 -- or Ottawa will do it for them.

Saskatchewan opposes a carbon tax.

The premier says memos obtained by an online publication show the federal government intends to tie a province's stance on carbon tax to equalization renegotiations scheduled for 2019.

Equalization is a program that transfers money from richer to poorer provinces so that they can offer government services at levels similar to elsewhere.