The crucial Copenhagen climate change summit could end without a deal, the US special envoy for climate change warned Saturday, while urging big developing economies to boost their efforts.

Todd Stern spoke before the Major Economies Forum featuring politicians from 17 countries met in London on Sunday.

This will help pave the way for a crunch summit in the Danish capital in December when nations will try to agree a new global climate treaty to replace the Kyoto Protocol which expires in 2012.

"It's certainly possible that there won't be a deal in Copenhagen," Stern told Britain's Channel Four television. "This is a tough negotiation.

"I think that the negotiations, if you look at what has been going on in the UN framework convention negotiations, the big negotiations of the 190 countries, have been making progress that is much too slow."

Stern added that major emerging economies were "doing significant things or (are) poised to do significant things", adding that had "never been true before."

"That's a very positive element," he added.

"What we need to have happen is for China and India and Brazil and South Africa and others to be willing to take what they're doing, boost it up some, and then be willing to put it in to an international agreement."

Stern called for "significant actions" from such countries.

"They don't have to do the same thing we have to do but they need to stand behind what their actions are and an international agreement just the way we do," he added.

"And if we can do that, then we can get a deal done, there is a deal there to be done."