Hopes that the world's biggest polluters will strike a deal to cut greenhouse gas emissions this year have been dashed by the outgoing United Nations official in charge of the talks.

Yvo de Boer, whose unexpected resignation as the UN's climate change chief this week dismayed policymakers and campaigners, said there was a “big question mark” over whether a meeting of the world's biggest economies in December would produce a treaty to tackle global warming.

Governments were hoping to adapt a limited accord signed two months ago at Copenhagen, which requires big cuts in emissions by 2020, into a fully fledged treaty at the crunch talks in Mexico. But Mr de Boer, who will leave in July, told the Financial Times yesterday: “I think you could get a decision at Cancun, and that what is agreed there [could be] turned into a treaty, but getting the big agreements on the content and the form at the same time, and finalising that in two weeks – that is a very heavy lift.”

His comments are a blow to hopes for a more substantial accord and provide further ammunition to those calling for the urgent reform or abandonment of the UN decision-making process.

“This resignation is simply dispiriting,” said Paul Bledsoe, policy director at the US National Commission on Energy Policy and a former White House adviser. “If someone as politically adept, dedicated and charismatic as Yvo de Boer can't bring the [UN] process to heel, then the process is broken and has to be reformed.”

Mr de Boer gave a crumb of comfort to supporters of a treaty, saying that businesses were taking up the challenge of climate change in spite of the failure of governments to draw up a legal framework on emissions.

Mr de Boer will leave for an advisory post with KPMG, the consultancy.