The revelations – made as the UN climate talks resume in Bonn [on 1 June] – come in Yvo de Boer's candid letter, written to colleagues days after the summit broke up in acrimony in December.

More than 130 world leaders had been persuaded by Britain and other countries to go to Denmark, where they were expected to put the finishing touches to a historic global agreement to limit carbon emissions, protect forests and put in place a mechanism to transfer billions of dollars from rich to poor countries each year. Instead, they arrived at a summit seething with mistrust.

According to De Boer, who will leave the UN framework convention on climate change (UNFCCC) in the next few months, the diplomatic debacle began to unfold when Denmark presented a one-sided draft agreement to a few select countries just before the start of the meeting.

The UN, aware that it was unbalanced and that it favoured the US and other richer countries, tried – but failed – to stop it.

"[The Danish text] destroyed two years of effort in one fell swoop," De Boer wrote. "All our attempts to prevent the paper happening failed. The meeting at which it was presented was unannounced and the paper unbalanced."

The paper was leaked to the Guardian, which had the effect of polarising countries' positions further, said the Danish journalist and climate change expert Per Meilstrup, whose book on the climate negotiations is published in Europe [on 1 June].

The presence of world leaders, such as Barack Obama and Wen Jiabao, further affected the talks, De Boer wrote. They had been expected to galvanize the summit and steamroller opposition to an agreement, but in fact proved counterproductive.

"Inviting heads of state seemed like a good idea. But it seriously backfired," he wrote. "Their early arrival did not have the catalytic effect that was hoped for. The process became paralysed. Rumour and intrigue took over."

Negotiators from more than 180 countries resumed the talks, hoping to make progress on key areas such as forests, finance and emission cuts.

But non-government groups warned that rich countries were already seeking to offer loans instead of grants to adapt to climate change.

"The $100 bn a year pledged by rich nations to help fight climate change could fail the poorest people if recent moves to deliver climate cash as loans continue," Oxfam's senior policy adviser, Antonio Hill, said. "At a time of economic emergency, when several poor countries are slashing critical health and education budgets to avoid a debt crisis, rich countries are considering saddling them with climate debt for a situation they did not cause. It's like crashing your neighbour's car and then offering a loan to cover the damages."

Friends of the Earth called for Europe to show global leadership by cutting its emissions by 40% by 2020.

"This is the minimum required if we're to have any chance of avoiding climate catastrophe and save the lives of the millions who will suffer devastating effects like more frequent storms, droughts and flooding," a spokesman said.