By David Ljunggren

OTTAWA (Reuters) - Canada is set to badly miss a 2020 target for cutting emissions of greenhouse gases, in part because of its failure to regulate the booming oil and gas sector, Parliament's environmental watchdog said on Tuesday.

The scathing report by Environment Commissioner Julie Gelfand will add to the political challenges faced by the right-leaning Conservative government, which polls show could lose power in an election set for 2015.

The government has deep political roots in energy-rich Western Canada - home to the Alberta tar sands - and says it will do nothing to harm economic development.

Gelfand found Ottawa did not even have a plan for how it would meet a commitment under the 2009 Copenhagen Accord to cut emissions by 17 percent below 2005 levels by 2020.

Environment Ministry figures show Canada will exceed the 2020 target by 20 percent if it does not take further steps.

"Federal measures currently in place will have little effect on emissions by 2020. ... There is strong evidence that Canada will not meet its international greenhouse gas emission reduction target," Gelfand said in an official audit covering the period between January 2011 and July 2014.

Critics, who have long bemoaned what they say is the Conservatives' lamentable record on the environment, say Canada has no chance of meeting its 2020 target of 612 megatonnes of total emissions of greenhouse gases. The Environment Ministry says 2020 output is on track to hit 734 megatonnes.

Environment Minister Leona Aglukkaq did not directly address Gelfand's prediction that Canada would miss the target, saying in a statement, "Our government is making significant progress on addressing climate change."

Emissions are growing fastest in the oil and gas sector, which the Conservatives have been promising to regulate for eight years.

The sector now accounts for more emissions than any other and is expected to contribute 200 megatonnes of emissions by 2020, or 27 percent of the overall amount.

"Despite this prediction, regulations for the sector have been repeatedly delayed," the audit said. The Conservatives, pressed on the delay, say they are consulting industry stakeholders.

Green activists cite Canada's failure to tackle soaring energy sector emissions as one of the reasons they want President Barack Obama to block TransCanada Corp's proposed Keystone XL pipeline from the Alberta tar sands in Western Canada to U.S. refineries on the Gulf Coast.

The Environmental Defence group said the report showed Ottawa's "unequivocal support for the tar sands and its vision of Canada as a fossil-fueled energy superpower is contributing to Canada's failure on climate."

Environment Ministry officials told auditors they were concerned the proposed oil and gas rules would make Canadian energy companies less competitive than their U.S. counterparts.

Gelfand said Ottawa did "not have a documented implementation plan for its own actions to reduce emissions ... there are no benchmarks against which to monitor and report on progress."

(Reporting by David Ljunggren; Editing by Peter Galloway and Leslie Adler)