Downing St has said its climate change targets are "ambitious" but it remains fully committed to renewable energy.
The EU wants to see an increased use of renewable energy
It was responding to a report in the Guardian that officials had told ministers the UK would miss EU targets by a wide margin.
Gordon Brown's official spokesman said: "It will be a major challenge not just for the UK but for the EU."
Tony Blair signed up to the EU targets in March - before he resigned as prime minister in June.
They include a 20% reduction in EU greenhouse gas emissions, compared with 1990 levels, or 30% if other developed nations agree to take similar action.
The targets also include an increase in the use of renewable energy, to 20% of all energy consumed, and a 20% increase in energy efficiency.
Mr Brown's spokesman said the EU's aims were "ambitious", but added that the UK was "on course to meet" its commitments under the Kyoto Protocol, which it ratified in 2002.
He added: "It is now for the EU Commission to propose how EU-wide targets should be met by member states."
However, the Guardian reported that it had seen an internal briefing paper for ministers from officials at the recently renamed Department for Business, Enterprise and Regulatory Reform.
This reportedly said that, under current policies, it would be "challenging" for Britain to reach a target of 9% for energy renewables such as wind, solar or hydropower - well short of the EU's 2020 target of 20%, but up from the current 2%.
According to the Guardian, the briefing paper said the UK had "achieved little so far on renewables".
The officials suggested ministers lobby other countries to get more flexible ways of reaching the targets, such as including nuclear power, the newspaper said.
But Energy Minister Malcolm Wicks strongly rejected claims that the government was not committed to renewables, saying various proposals to meet the targets had been drawn up for discussion.
"At the moment, about 4 to 5% of our electricity comes from renewables. We're on course for that to be three times as much - 15% by 2015.
"We've now got this more demanding European target, in other words, not just electricity, but fuel we need for our cars and our heating as well, and the issue is how do we get there?"
He added: "We are determined to make our contribution on this... We are moving steadily, year after year, in the right direction.
"We're ambitious about renewables and I'm absolutely determined that we move forward on renewables."
But environmental group Friends of the Earth said the briefing paper showed the government was trying to water down its commitments.
Campaigns director Mike Childs told BBC Radio 4's Today programme: "It's very clear that what they're trying to say is 'we want to introduce maximum flexibility within the targets', and they float ideas about saying 'can we use our nuclear power as part of that targets?
"Now, clearly nuclear power isn't a renewable industry, so what they are trying to do is weaken it as far as possible.
"It is a very damning document and it does give a very interesting insight into the way that government operates."
Shadow business, enterprise and regulatory reform secretary Alan Duncan accused the government of "living a lie" over climate change.
Liberal Democrat environment spokesman Chris Huhne said: "This leaked paper shows that ministers are not trying to meet their renewables targets but instead are trying to wriggle out of them."