The Met Office headquarters was today named as one of Britain's worst public buildings for pollution due to a £30 million supercomputer used to predict climate change.

The giant IBM machine fills two special halls the size of two football pitches and is believed to be the second most powerful system in the UK.

It produces 12,000 tonnes of carbon dioxide a year and the Met Office HQ, in Exeter, Devon was named as one of the worst polluters in a new green league table published by the Department of Communities and Local Government.

Met Office spokesman Barry Gromett defended the computer and said it was needed to accurately predict the weather and its predictions helped to reduce global carbon emissions.

He said most of the building has 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.

"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."

By 2011, the computer is anticipated to have a peak performance approaching 1 PetaFlop - equivalent to more than 100,000 PCs and over 30 times more powerful than what is currently in place.

It will offer 15 million megabytes of memory and require 1.2 megawatts of energy to run.

Mr Gromett said the extra power will mean they can achieve more-accurate and detailed short-range weather forecasts through "high-resolution computer simulations".

He said the system is also being used for research on climate change and its impacts on society and the economy.

Friends of the Earth spokesman Maurice Spurway said: "Life is full of ironies and I think this is one of those situations."