Tony Blair said today it was time for the world to get moving as negotiators in Copenhagen entered the final few days of talks still deeply divided on how to avert climate catastrophe.

With ministers, tycoons and celebrities flooding to the Danish capital, the former British prime minister said a compromise was possible, though likely to fall short of the actions recommended by scientists to avoid dangerous levels of global warming.

"There can be a deal at Copenhagen. There should be a deal. It will not be all that everyone wants. But it was never going to be," he told an audience on the sidelines of the conference.

But time is running out. Major differences emerged in the first week of talks as developing nations accused richer countries of offering insufficient emissions cuts and inadequate financial and technological assistance to help poorer countries deal with climate change.

Heads of state will fly in at the end of the week to try to seal a deal. But even that is emerging as a contentious issue because developing nations want an agreement in place before they arrive and have their arms twisted.

Ed Miliband, energy and climate change secretary for Britain, warned that far more progress needed to be made before the leaders arrive.

"We're getting close to midnight in this negotiation and we need to act like it. That means more urgency to solve problems not just identify them, more willingness to shift from entrenched positions and more ambitious commitments," he said.

Blair, who is here not as a negotiator but as head of the climate group NGO, said the key was to put changes in place now and build momentum later.

"Let us get it under way. And that really is the objective for Copenhagen: to get us moving: To be the signal set that makes us switch track to low carbon future.

"And to make sure everyone is on the train, going in the same direction. Some will be at the front, some at the back. Some will pay more than others, but together we are on board for a new destination for the global economy."

Blair was prime minister when the world's last climate agreement – the Kyoto protocol – was signed in 1997 after heads were banged together by negotiators at the last minute.

Twelve years on, Blair acknowledged the flaws of that deal.

"Kyoto was a treaty that aimed at making a point, but less successful at making a policy. Copenhagen is where we need to make a policy. It is time to lift this issue firmly within the framework of a credible, achievable policy for change."

Despite the leak of damaging emails from the University of East Anglia Blair said the need for action was as evident as ever. "It is said that the science around climate change is not as certain as its proponents allege. It doesn't need to be.

"What is beyond debate, is that there is a huge amount of scientific support for the view that the climate is changing and as a result of human activity. Therefore, given the seriousness of the consequences, we should act."

But the scientists advice is not going to be fully acted upon. The Intergovernmental Panel on Climate Change – a body of the world's leading scientists – has called for deep cuts in emissions from rich nations and a significant move away from business as usual by developing countries in order to keep global warming to two degrees Celsius, the level at which climate change will start to hit catastrophic proportions.

Given everything that is currently on the table, Blair said, the developed world would reduce emissions by just under 20% by 2020 from 1990 levels. There was also a danger, he said, that rising greenhouse gas emissions from developing nations could eclipse efforts made in richer nations.

He said targets could be revised and strengthened later, it was important to send a signal to the business and science community that they should invest and develop low-carbon technology.