Brazil and France have agreed a common position on fighting global warming before next month's UN climate change conference in Copenhagen.
They will pursue the goal of reducing industrialised nations' emissions to 50% below 1990 levels by 2050.
French President Nicolas Sarkozy and Brazilian President Luiz Inacio Lula da Silva announced the move in Paris.
It comes days after Britain said it was highly unlikely that a legally binding climate treaty can be agreed this year.
The climate conference, in the Danish capital from 7 to 18 December, aims to create a successor to the 1997 Kyoto treaty limiting carbon emissions.
But British Climate Secretary Ed Miliband said only a political deal was likely.
Developing countries reacted with frustration and disappointment to his comments.
Negotiations so far have been clouded by disputes between rich and developing nations, particularly about who will be financing a new climate deal.
Last week, finance ministers from G20 countries said they "recognised the need to increase significantly and urgently the scale and predictability of finance" and "that finance will play an important role in the delivery of the outcome at Copenhagen".
But on new ideas on how to raise money and manage it - and of new commitments - there was nothing, the BBC's environment correspondent Richard Black says.
At a news conference in Paris, Mr Sarkozy praised Brazil for being "the first developing country to put its proposals on the table", and urged on other nations to follow its example going into the Copenhagen conference.
President Lula said the document he had signed with Mr Sarkozy was "more than a declaration of intent, it is a climate bible".
It comes after Brazil said it aimed to cut its carbon emissions by at least 36% by the year 2020.
Amid the ongoing deadlock in negotiations preceding the conference, both presidents said they would attend the Copenhagen summit, and urged the United States and China to get behind strong measures.
The two leaders said they would try to drum up wider support for their initiative before the meeting.