The Met Office building is rated G in a league table A to G
A £30m computer for predicting climate change is making the Met Office's Devon headquarters one of Britain's worst public buildings for pollution.
The IBM machine fills two special halls the size of football pitches.
It produces 12,000 tonnes of carbon dioxide a year, making the building among the worst in a Department of Communities and Local Government table.
The Met Office said it saw the irony of the situation, but the computer was needed for weather predictions.
The Met Office building is rated G in a league table which rates buildings A to G according to their energy efficiency.
Met Office spokesman Barry Gromett said most of the building had an excellent green rating.
Mr Gromett said: "Our supercomputer is vital for predictions of weather and climate change.
"By failing to discriminate between office and supercomputing facilities the process reflects badly on the entire Met Office site.
The computers take in weather data from around the world
"In fact the general office space is rated excellent and has consistently done so since the Met Office building in Exeter was completed in 2003."
The IBM supercomputer can do more than 100 trillion calculations a second, allowing it to take in hundreds of thousands of weather observations from around the world.
Friends of the Earth spokesman Maurice Spurway said: "Life is full of ironies and I think this is one of those situations."
Thousands of buildings, including Exeter University, have been rated in the league table obtained under the Freedom of Information Act.
It said that despite poor ratings at several of its sites, it had reduced its carbon footprint by 10% in the past few years.