The thermal image of the EDF building shows heat loss in red

BBC South East used a thermal imaging camera to show heat escaping from doors, windows and air conditioning vents at EDF Energy in Maidstone.

But its website advises its 5.1 million customers to save energy at home.

"Obviously, this building is not as energy efficient as we would like it to be," said spokesman Gareth Wynn.

The company advises people to put draught strips around windows and outside doors, insulate lofts and install cavity wall insulation.

It also recommends using low-energy light bulbs, and turning off TVs and hi-fi equipment rather than leaving them on standby.

Staff in EDF's Maidstone offices are housed in a 1960s building

"It would be nice if they practised what they preached," said thermal imaging expert Stuart Little.

"It would be nice to see guys like EDF in energy efficient, modern, well-built buildings and not in 1960s blocks like this."

EDF said it was taking steps to improve the insulation of its walls and windows.

"We have set ourselves a target that by 2010 we will have reduced carbon emissions linked to energy consumption by at least 15%," said Mr Wynn.

"That is going to mean addressing some of the points identified - at the moment it is not where we would like it to be."