A new G8 deal on climate change, halving global emissions by 2050, was last night hailed as "major progress" by Gordon Brown, but other nations said it lacked ambition and green campaigners dismissed it as "pathetic".

A new G8 deal on climate change, halving global emissions by 2050, was last night hailed as "major progress" by Gordon Brown, but other nations said it lacked ambition and green campaigners dismissed it as "pathetic".

The five-page communique spoke of the "vision" to reach the 50% target by 2050, slightly hardening the previous pledge just to "seriously consider" emission cuts.

However, the world's leading industrialised nations stressed: "This global challenge can only be met by a global response, in particular, by the contributions from all major economies."

The agreement comes ahead of a major UN summit next year in Copenhagen to find a replacement for the Kyoto Protocol, which expires in 2012.

"There has been major progress on the climate- change agenda, beyond what people thought possible a few months ago," declared the Prime Minister; the pledge to halve emissions by 2050 had been previously opposed by US President George W Bush.

Mr Brown noted: "Countries which previously objected to setting overall targets have accepted these targets subject to there being an internat ional agreement."

Angela Merkel, the German Chancellor, described it as a "significant step forward" while Mr Bush insisted it represented "substantial progress".

However, five of the largest emerging countries - Mexico, Brazil, China, India and South Africa, the so-called G5 - had wanted the G8 to go further and cut greenhouse emissions by more than 80% by 2050.

In a joint statement, the G5 said: "It is essential that developed countries take the lead in achieving ambitious and absolute greenhouse gas emission reductions."

South Africa said the G8 statement was a "regression", criticising the lack of firm targets to achieve sufficient cuts in emissions.

RK Pachauri, Chairman of the UN's Intergovernmental Panel on Climate Change, challenged the developed countries to show leadership.

"They should get off the backs of India and China. They should say: We'll assist you to move to a pattern of development which is sustainable, low in terms of emissions intensity but we as the richest nations are willing to take the lead and we affirm our commitment to do so.'"

Meantime, John Sauven, Greenpeace executive director, who said setting "ambiguous long-term targets"

for all countries instead of tough ones on themselves showed the leaders were engaged in "a festival of vacuous back-slapping that bore no relation to the scientific reality we face". He added: "Thank God this was Bush's last G8."

Kim Carstensen, of the WWF's Global Climate Initiative, added: "If after a year's work all you have is a shared vision' instead of seriously considering', it's pretty pathetic."

Elsewhere in what was regarded as a victory for Mr Brown, fellow leaders also insisted they would not abandon a pledge made at the Gleneagles summit in 2005 to double development aid by 2010 - half of that going to Africa.

They also agreed to spend more than £70bn on malaria nets, school places, modernising agriculture and combating infectious diseases.

"We are delivering everything we promised; other countries will do the same," said the Prime Minister.

"Ten million children will go to school. Malaria nets will save thousands and more healthcare. These are big concrete changes," he said.

Today, the global economy will come back under the spotlight when the G8 leaders are joined by their counterparts from eight big emerging economies, including China and India, for talks.

The summit is taking place at the luxury Windsor Hotel on a mountain top above spectacular Lake Toya amid very tight security, the cost of which - some £283m - has sparked controversy in Japan and beyond.