"I'm reasonably optimistic that this Copenhagen (summit) will be a very important milestone," Ban told reporters after talks with British Prime Minister Gordon Brown in London.

"At the same time, realistically speaking, we may not be able to have all the words on detailed matters."

He added: "We need the political will, if there is a political will I'm sure there is a way we can conclude a binding agreement."

Brown said he thought a deal was possible and restated his pledge to attend the talks in person.

"We will leave no stone unturned in the next few weeks," he said.

"We believe it's possible to get an agreement on long-term targets and intermediate targets."

Hopes for a legally binding agreement on slashing carbon pollution by the United Nations conference in Copenhagen in December are fading, and diplomats have instead set their sights on a political deal, sources say.

Negotiators from more than 190 countries are meeting in Barcelona this week for a final round of talks before the December 7-18 conference in Denmark.

The aim is to craft a new pact on climate change beyond 2012 but the negotiations are mired in discord over how to apportion carbon emission cuts and finance a switch to lower-pollution technology.

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