MONTREAL - Prime Minister Justin Trudeau says Ontario and other provinces trying to block the federal carbon tax are shortsighted and irresponsible.

Trudeau gave a speech to an environmental conference in Montreal today during which he slammed Ontario's constitutional challenge to the federal Greenhouse Gas Pollution Pricing Act.

Manitoba, Saskatchewan, Ontario and New Brunswick are all either challenging the carbon tax in court or have signalled an intent to do so.

Following Trudeau's brief address, he was met on stage by high-profile Quebec environmentalist Steven Guilbeault for an armchair discussion.

Guilbeault, who is currently advising the federal government on ways to address climate change, steered clear of contentious issues in his questions.

The ecologist has acknowledged he is being courted by the Liberals to run for the party in the October federal election. He told reporters he has been approached by more than one political party and hasn't decided whether to run for office.