Hopes of reaching an international deal on curbing global warming were boosted this weekend by the announcement that more than half the world's leaders would attend the Copenhagen summit starting tomorrow.

A stream of acceptances has taken the number going to the conference to more than 100 – and more could decide to attend.

"The presence of so many leaders doesn't guarantee success but it makes failure harder. They won't want to come away without an agreement," a source close to the negotiations said.

The Danish government, which issued 191 invitations to heads of state and government, announced that more than 100 had accepted. They will attend on the last two days of the talks on 17 and 18 December.

Early sessions at the United Nations summit will be conducted by officials from participating governments, but parallel contacts between leaders will take place at the same time.

Britain says the high levels of participation and engagement by heads of state in the Copenhagen negotiations contrasts with the stalled international trade talks in which few world leaders participated.

Divisions between – and within – nations on the size of the problem and the cost of tackling it mean that the chances of negotiating a binding international treaty on climate change at Copenhagen look remote.

However, cautious optimism that an agreement can be struck has grown as most of the world's biggest economies have said they were prepared to commit themselves to targets on carbon emissions. The United States, China, India, Brazil and Indonesia have all put offers on the table, with an announcement expected shortly from Mexico.

Opposition to cutting emissions has come from Saudi Arabia, which has seized on leaked emails from the University of East Anglia's Climatic Research Unit that appeared to cast doubt on the evidence of man-made global warming. But negotiators believe the Saudi stance is designed to protect its oil industry and the kingdom will eventually be forced into line by the weight of world opinion.

British PM Gordon Brown was the first to announce his participation. Those joining him include taoiseach Brian Cowen, French president Nicolas Sarkozy, German chancellor Angela Merkel, Chinese premier Wen Jiabao and Japanese Prime Minister Yukio Hatoyama. The Indian prime minister, Manmohan Singh, has not disclosed whether he will attend.

Barack Obama has said he will attend the negotiations next Wednesday, but hopes are rising he could return
the following week.

The scale of the task ahead in the hugely complex negotiations was underlined by Erik Solheim, the Norwegian environment minister.

He said: "These are the most difficult talks ever embarked upon by humanity. The effects will be felt by the rice farmer in Sichuan in China, by Google headquarters in Seattle, or by the oil worker in Norway."