OTTAWA - Prime Minister Stephen Harper has changed course and will attend a United Nations conference next month with some 65 other world leaders, despite asserting no global deal on climate change is imminently achievable.

The Conservative government has consistently played down expectations for the Copenhagen conference, where it was hoped the global community would agree on a post-2012 accord to reduce greenhouse-gas emissions.

Earlier this month, Danish Prime Minister Lars Lokke Rasmussen formally invited 191 government leaders and heads of state to Denmark to push along the negotiations for a successor treaty to the Kyoto Protocol.

Other Western leaders, including British Prime Minister Gordon Brown and Australia's Kevin Rudd, quickly got on board.

But the entreaties were rebuffed by Harper -- at least until Washington announced Wednesday that U.S. President Barack Obama would be stopping in on the conference next month. Chinese Premier Wen Jiabao has also announced his participation.

Within 24 hours, Harper spokesman Dimitri Soudas announced the prime minister had decided to attend Copenhagen because a "critical mass" of leaders is now going.

The announcement early Thursday came as the prime minister boarded a plane for the Commonwealth Conference, in Trinidad and Tobago, leaving followup questions on the decision hanging at 35,000 feet.

Obama will attend Day 3 of the Dec. 7-18 conference, not the leaders' segment slated for the final two days. That has some environmental activists criticizing what they say will simply be an Obama photo-op in Copenhagen.

It was not immediately evident when Harper will attend the conference.

The prime minister had brushed off a question in the House of Commons on Wednesday about attending the summit. A day later, Environment Minister Jim Prentice was maintaining that Harper has "been very clear that he will go to Copenhagen, that he will participate in Copenhagen. The exact time at which he will be participating hasn't yet been determined."

Transport Minister John Baird told the House that "Canada is committed to a successful climate-change outcome in Copenhagen. ... Even the prime minister will be there to forcefully argue for a strong agreement."

The sudden rewrite of Conservative messaging signals a profound change in tone from the deep skepticism Harper expressed only a week ago at an APEC summit in Singapore.

Harper said at APEC that the assembled leaders shared "a pretty strong consensus ... that the countries of the world remain a long way from a binding, legal treaty on climate change."

He also said it may be time to "get our negotiators out of this morass of hundreds of pages and thousands of brackets of (negotiating document) text and into looking at the big picture and coming to some agreement on some big-picture items."

But with UN Secretary General Ban Ki-moon and Rasmussen both attending this weekend's Commonwealth summit to push the climate issue, Harper may have felt pressure to put a more tangible, less nebulous face on Canada's response.

An international coalition of environmental groups on Thursday proposed suspending Canada from the Commonwealth because its "lack of action on climate change is contributing to droughts, floods and sea-level rises in small island states and vulnerable Commonwealth countries such as Bangladesh."

And a rather more restrained collection of five scientific organizations sent an open letter to parliamentarians urging Canada "to act responsibly. We must act now."

A number of provincial environment ministers are committed to attending Copenhagen and their participation has also threatened to embarrass the Harper government internationally.

"We are not out to embarrass the federal government at an international setting," Ontario Environment Minister John Gerretsen told The Canadian Press this week.

"But on the other hand, we also want to join with other like-minded subnationals that, in effect, have been more proactive on the whole climate-change agenda over the last couple of years."

In Quebec City, Liberal Leader Michael Ignatieff dripped scorn on Harper's about-face on the Denmark conference.

"What a coincidence," said Ignatieff. "He's been dragged there by the decision of the president to go there."

Conservatives, however, are not prepared to take all the heat for Canada's long, sorry record on climate change measures.

Prentice made a point of raising the 1997 Kyoto Protocol in which the Chretien Liberals committed to deep carbon reductions the country never came close to meeting.

"One thing the Conservative government will never do is fly over to Copenhagen, pull a target out of the air that is ill-suited to our industrial base, to our geography and agree to damaging the Canadian economy," Prentice told the Commons.

"That will not happen on our watch."