Subject to formal endorsement in summit conclusions being prepared in Brussels, Europe has agreed to make a conditional offer to the rest of the world at global environment negotiations in Copenhagen in December.

The move is a victory for the Prime Minister, who yesterday warned the summit that failure to include figures would risk the breakdown of the UN talks.

German Chancellor Angela Merkel said the financial cost should not be included in the EU’s proposed package of ambitious climate change targets – at least not until other nations signal their readiness to pay their share.

But the summit text puts a 100 billion euros-a-year (£89.6bn) price on curbing global warming by 2020, of which annual public funding is estimated at 22-50bn euros.(£19.7bn – £44.8bn.)

The EU’s combined share of that would be between 7-10bn euros (£6.2bn – £8.96bn) a year by 2020.

UK officials say that, subject to the “conditional offer” being accepted in Copenhagen, the UK share of the EU contribution would work out at about £1bn a year by 2020.

Mr Brown had wanted a narrower range of 30-40bn euros (£26.8bn – £35.8bn) as the global public funding estimate to keeping global warming below a two degree rise, but, to bring Ms Merkel on board, he accepted the need for a looser range of figures.

“I think that this is a breakthrough that takes us forward to Copenhagen and makes a Copenhagen agreement possible.” said the Prime Minister.

“Europe is making three conditional offers – money on the table, saying we will do everything we can to make a climate change agreement happen, and help for developing countries into that agreement.

“Now we want other countries to respond to what we’re doing.

“I think developing countries can now say they are ready to cut their emissions substantially over the next few years.”

Mr Brown also wants to up the ante by pressing the world to start making financial contributions to the climate change costs earlier than the planned start-date of 2013.

Additional figures added to the EU offer today call for global pre-2013 spending on climate change of 5-7bn euros (£4.5bn – £6.2bn).

A UK government official said: “All of this is conditional on other countries putting in their fair share”.