Oxford City and Oxfordshire County Councils spent nearly £5m between them on electricity in 2005/2006.

The bill includes lighting buildings, operating traffic lights and illuminating road signs.

Now staff at the councils have been told to play their part in slashing the £4.9m bill by turning off their computers and lights when they leave offices.

The county council - the highways authority - has also set itself a target of switching off the lights on 100 street signs a year, where regulations permit, to save money.

City councillor Susan Roaf, an expert on climate change and its impact on the built environment, said: "I am gobsmacked (at the bill), but I always said the council didn't realise what a timebomb it was sitting on.

"The fundamental problem we have to understand is these buildings are a major source of expense so we have to put together a programme for their improvement - it's not rocket science."

The county council is involved in an energy reduction programme in schools across Oxfordshire where pupils are taught about managing and monitoring energy use.

But it has been recognised that work also needs to be done in council buildings.

The city council is currently having its buildings assessed for energy efficiency, while County Hall already employs property consultants to provide an "energy management service". Mrs Roaf, a visiting professor from Arizona State University in the US, believes climate change will have a huge impact on councils and their work.

In the future, she thinks old people's homes that face south or west will have to have awnings erected to protect them from the sun.