Their comments after EU ambassadors met in Brussels confirmed the 27-nation bloc's commitment to unilateral target carbon dioxide emissions to 20% below 1990 levels over the next decade.
Some EU countries such as Poland, Italy, Cyprus and Malta had opposed making the more ambitious conditional offer over concerns that it would be too costly for industry.
"Italy and Poland said at the meeting that they were concerned but they wouldn't stand in the way," an EU envoy said.
Before United Nations-sponsored climate talks in Copenhagen in December, the EU offered to deepen its cuts to 30% of 1990 levels if other rich countries made similar efforts.
Ambassadors agreed the EU should sign up to the accord with the 20% cuts in a letter to be sent to the UN on Thursday, but that the 30% conditional offer should still be made, even if the conditions behind it are far from being met.
The meeting in the Danish capital ended without agreement on binding cuts to climate-warming carbon dioxide emissions, leaving countries until 31 January to submit their own plans.
Experts say the total cuts offered there by rich countries amount to no more than 18% and fall far short of the 25-40% that UN scientists outline as necessary to avert dangerous climate change.
The world is on track for temperatures to rise to 3.5 degrees Celsius above pre-industrial levels by the end of this century, which would bring catastrophic melting of ice sheets and rising seas, some scientists say.
Britain, Denmark and the Netherlands were among the countries that defended the 30% offer.
"The UK remains committed to the conditional offer of 30% to stay on the table to ensure that we do not lose the momentum that has been generated over the last few months," said an official from Britain's Department of Energy and Climate Change.
(EurActiv with Reuters.)
