The president committed America for the first time to setting targets to cut its own greenhouse gas emissions.

And he called for a global warming summit later this year to seek a new international consensus.

Mr Blair, speaking on a visit to South Africa, told Sky News: "This is what we have been working for all the way through because the important thing is we need a deal in which America as well as China and India - in which all these countries are involved for the first time - and a deal that's about cutting these dangerous emissions. Obviously it's a big step forward."

Referring to next week's G8 summit in Germany the Prime Minister went on: "It sets the right framework for next week's meeting.

"I think the important thing is, for the first time, America's saying it wants to be part of a global deal.

"For the first time it's setting its own domestic targets. For the first time it's saying it wants a global target for the reduction of emissions and therefore for the first time I think the opportunity for a proper global deal.

"I think what is important is for America to come into this and offer real leadership with the same common goal everyone else has.

"If America's not part of this agreement nothing's going to happen; China has to be part of this agreement."

"The significance of this is America accepting this is a real problem now, accepting it's got to offer real leadership on this issue and being prepared to be part of a global deal at the heart of which will be the reduction of emissions."