There is still "a lot of work to be done" to secure any international deal on climate change, Gordon Brown has said.
He spoke as an apparent dispute between China and the United States threatened to leave the Copenhagen talks deadlocked.
The British Prime Minister has been locked in frantic negotiations all day in a bid to secure global agreement in the face of Chinese opposition to elements of a proposed deal.
As the impasse threatened to send the talks into an unscheduled second day, Mr Brown said he was still hopeful of unblocking the two remaining issues in dispute.
But he revealed he had prepared a back-up plan - believed to involve talks between a smaller group of nations - in case the latest session ended in failure.
Speaking during a brief pause in the talks, he told reporters: "There are two issues that are still outstanding. They are issues that have got a lot of work to be done on."
One sticking point is over whether a long-term goal should be to limit temperature rises to 2C but the Chinese are sensitive to calls for monitoring of each country's efforts to meet commitments.
He spoke out as it appeared one of the most serious barriers to progress, an apparent dispute between China and the US, could have been eased after the White House confirmed a fresh meeting had been set up between President Barack Obama and Premier Wen Jiabao.
Earlier, a clearly frustrated President Barack Obama displayed impatience with world leaders' failure to reach a new climate accord.
He urged them to accept a less-than-perfect pact but offered no new US concessions.