Emissions of the greenhouse gas carbon dioxide rose in the European Union by 1.5% in 2003 after falling in 2002, the European Environment Agency reports.
Environmentalists say the news is a setback in plans to tackle climate change
Italy, Finland and the UK were named as the worst offenders while cold weather was blamed for a rise in the use of fossil fuels to heat homes and offices.
Some commentators now doubt the EU can meet its promise to cut emissions by 8% of 1990 levels by 2012.
A spokesman for Friends of the Earth called the new figures "shocking".
"The blame goes mostly to national economy and industry ministers, who constantly block any attempts to introduce mandatory targets for renewable energies, energy efficiency rules or fuel consumption standards for cars," Jan Kowalzig said.
Carbon dioxide emissions have risen by 3.4% since 1990, according to the EEA figures.
The Copenhagen-based EEA said emissions in the 15 old EU member states increased by 53 million tonnes, or 1.3%, in 2003, after a drop in 2002.
According to its figures, between 2002 and 2003, Italy, Finland and the UK saw the largest emission increases in absolute terms - 15 million tonnes, eight million tonnes and seven million tonnes respectively.
EU Environment Commissioner Stavros Dimas called on member states to meet their commitments.