A one-sentence note from China's top climate change negotiator, Su Wei, authorized the addition of China to the list attached to the nonbinding accord brokered by President Barack Obama in the final hours of the December climate change summit in the Danish capital.
India sent a note on Monday that it "stands by the contents of the accord."
More than 100 countries had earlier replied to a query by Denmark whether they wanted to be "associated" with the accord.
The delayed response by the world's two fastest growing polluters had raised concerns that without their concurrence the accord could fall apart.
Now, their avoidance of the word "associated" was being seen as deliberate and possibly a step to distance themselves from full endorsement.
The responses to the Danish question highlight the gulf that remains after the disappointing conference in Copenhagen. The summit fell well short of its original ambition of a legally binding treaty controlling the world's emissions of carbon dioxide and other gases blamed for global warming.
The accord, concluded in a flurry of last-minute diplomacy, set a goal of limiting the increase in the Earth's average temperature to 2 degrees Celsius (3.8 Fahrenheit) from preindustrial levels. But it did not say how that can be achieved or how countries should share the burden of cutting carbon emissions.
It also said developing countries should be given $30 billion over the next three years to help them cope with changes already occurring in rainfall patterns and other effects of climate change. So far, no proposals are on the table for raising or distributing those funds.
The European Union's climate commissioner, Connie Hedegaard, told the EU parliament in Brussels on Tuesday it would be risky to expect a legally binding deal to emerge from the next climate summit in Cancun, Mexico.
Hedegaard told the European Parliament that "remaining differences between parties may delay agreement on this until next year."
Among those differences are whether the U.N. negotiations among 194 countries, bogged down in cumbersome committees producing highly contentious draft texts, should be the primary avenue for reaching a deal.
The United States has said the Copenhagen Accord should be taken as the basis for negotiations leading up to a full legal agreement in Mexico.
In its note this week, India strongly disagreed. "The Accord is not a new track of negotiations or a template for outcomes," said India's environment secretary Rajani Ranjan Rashmi.
In a letter dated in January, Chinese Premier Wen Jiaboa also said "it is neither viable nor acceptable to start a new negotiating process" outside the framework of previous U.N. treaties.
Wen's letter, however, praised the Copenhagen Accord in warmer tones than the official note received Tuesday. He said China "highly commends and supports" the December deal.