"At the moment, instead of a 20 percent reduction we will achieve a 14 percent reduction," Blair told parliament after the government launched a review of its climate change policy.
"However, we have years to go before we have to achieve that target ?nd as we say today, we don’t accept we won’t meet it. We’ve got to make sure that we take the measures to meet it."
He added that Britain would, however, meet its more modest Kyoto protocol target of cutting emissions by 12.5 percent from 1990 levels by 2008-2012.
Blair has said he wants to make climate change a priority during Britain’s presidency of the G8 group of leading nations and the European Union next year.
Environmental groups said Britain needed to act urgently if it hoped to persuade other countries, like the United States, to curb greenhouse gas emissions.
"The government must make significant cuts in UK carbon dioxide emissions if Tony Blair really wants to lead the world in tackling climate change," said Tony Juniper, director of Friends of the Earth.
The group called for more investment in cleaner power generation to replace ageing coal-fired plants as well as measures to improve energy efficiency and encourage less polluting transport options.
Britain is relying on a threefold expansion of renewable energy, greater energy efficiency and curbs on CO2 emissions from industry to meet its 2010 target.
Emissions fell in the 1990s as coal-fired power stations were replaced by cleaner gas ones, but they have risen in the past couple of years partly as generators burned more coal after a rise in gas prices.
Industry has been set limits on CO2 pollution under the first phase of the European carbon emissions trading scheme which starts in January.
If companies exceed their limits, they have to buy quotas from firms which undershoot their targets.
As part of the review, the government said it was considering its approach to the second phase of the EU’s carbon emission trading scheme which runs from 2008- 2012.
In October, Britain said it would increase carbon dioxide quotas for industry after complaints — especially from the power sector — that its CO2 reduction plans were too tough.
The government argues that its first proposal for cuts under the EU scheme was based on incorrect data and the new quotas will force industry to cut emissions substantially.