But the National Roundtable on the Environment and the Economy said important parts of the plan were vague and exaggerated the extent of emissions cuts.

In a report released late on Friday, the group pointed to inconsistencies in the government's calculations and said some of the data was either too murky or too skimpy.

It said that while the plan would result in "significant emissions reductions and contributions to future emissions reductions," the methodology used to calculate the cuts "represents an important inconsistency in accounting for emissions reductions."

Green groups and opposition parties say Canada should stick to its Kyoto targets, which call for a 6 percent cut in Canadian emissions from 1990 levels by 2012.

Data for 2004, the latest year available, shows emissions were around 35 percent above the 2012 Kyoto target. Much of the increase took place while the previous Liberal government was in power, from late 1993 to early 2006.

Environment Minister John Baird, accusing the Liberals of ignoring the problem, noted that the report agreed Ottawa's plan would result in major cuts in greenhouse gas emissions.

"This will take decades ... . We're working every single day to push this file forwards," he told Reuters in an interview. "The enormity of the task won't be met by putting out one plan on one day."

Environmentalists say even if the new plan meets all its targets, emissions in 2020 will be 11 percent above where they should have been in 2012 under Kyoto.

Harper, who favors cutting emissions at a gentler pace, is due to speak about climate change on the sidelines of the UN General Assembly in New York on Monday.