Homes in the north of England are more energy efficient than those in the south, according to a survey released today. Analysis carried out by British Gas found all 10 of the least efficient areas were based in or around London, with Camden being the worst. Eight of the top 10 most energy efficient regions were in the north with Knowsley, in Merseyside, the best in the country.

The Energy Savers report was based on energy audits which graded one million households around the UK from A to F, with A being the most efficient.

The average grade in the audit was a D, with 65% of all homes rated D or worse. The overall rating in Camden was D minus. The audits were carried out along the same lines as the energy efficiency reports that will become compulsory for all properties on the market next year.

A spokesman for British Gas said the biggest single failing point for households was a lack of cavity wall insulation, with many properties built between the 1930s and 1990s losing heat through outside walls. Other factors that influenced a property's score included whether or not it had double glazing and whether energy efficient appliances and lightbulbs were installed.

The spokesman said that while the age of housing stock made a difference to the energy efficiency of different areas, the main reason why some regions rated more highly was government investment. "There have been a number of targeted schemes in some northern areas. The government has created warm zones in places like Darlington and Hartlepool and has targeted a significant amount of money into these areas to improve energy efficiency," he said.

Hartlepool was fourth in the British Gas ratings. Other factors behind the north-south divide were likely to be temperature, with those in the cooler north more likely to install insulation, and wealth, with those who spent more of their income on energy more likely to do what they can to cut costs, he said.

Wealthy areas such as Kingston upon Thames, Westminster, and Kensington and Chelsea all made it into the survey's top 10. The mayor of London, Ken Livingstone, said households in the capital should take small actions like using energy efficient bulbs and turning off appliances to reduce energy use.

"Around 40% of carbon emissions in London come from energy use in homes. There is a lot that individuals can do in their own homes to cut their contribution to climate change and save themselves money in the process," he said. "We probably have about a decade to avert catastrophic climate change. The decisions each of us now take could make a real difference."

Last week it was announced that the mayor's riverside base, City Hall, would be fitted with solar panels to reduce energy use. British Gas today launched a "save a billion" campaign to raise awareness of the scale of UK energy waste and inform people how to make their homes more energy efficient. Amanda Mackenzie, the brand director for British Gas, said: "One in every three pounds spent on energy in the UK is wasted through poor energy efficiency.

"Energy waste is a global issue but if we all do our bit to reduce waste then we can permanently cut our energy bills and help to save the environment." Advice includes turning down thermostats by 1 degree centigrade, which could save a household £49 a year, and turning off appliances rather than putting them on standby, an action which could reduce bills by £46. Households can still sign up for an energy audit with British Gas hoping to have results for two million properties by Christmas, which it will feed into an online database.