Heat-sensing vehicles are being sent out after dusk to photograph thousands of homes.

Thermal images showing heat loss from properties are then posted out to householders, highlighting poor insulation in an attempt to shock them into action.

The HeatSeekers scheme, run by the Energy Saving Partnership alongside local authorities, then offers to arrange for insulation work to be carried out, getting a cut of the fee from the contractors.

Some 20,000 homes have been photographed with the backing of Charnwood Borough Council and another 47,000 are due to be checked next winter.

Blaby and North West Leicestershire district councils are to start similar schemes.

Energy Saving Partnership managing director Keith Hewitson said: "We use top-of-the-range cameras fitted inside our vehicles.

"It's the kind of technology used by the military and emergency services.

"We're not taking a typical photograph, we're taking a thermal image where each pixel represents a temperature.

"Higher temperatures are represented by red shades, while lower temperatures are represented by blue colours.

"It doesn't see through walls or into anyone's home so there's no privacy issue.

"We are just checking the temperatures."

The camera vans have been cruising down streets at 8mph. They can photograph up to 1,000 homes every hour as they travel along streets between 7pm and midnight.

Work must be done in winter because the equipment only works when there is a large difference between outdoor and indoor temperatures.

Councils are joining forces with the partnership to help encourage their residents to be more environmentally-friendly.

They do not receive any money from the scheme.

A Charnwood Borough Council spokesman said: "The aim is to identify those dwellings in need of loft, or cavity wall insulation."

Two trained thermologists are in each monitoring vehicle, with one driving and one checking the images are accurate.

But some residents said they are not convinced the images are precise.

James Munroe, 76, from East Goscote, said: "I've got cavity wall insulation and the recommended amount of roof insulation, yet the photos I received indicate my home is giving off high levels of heat.

"Even three cars and the lawn in front of my property look to be giving off heat.

"I wonder how reliable these images are."

But Mr Hewitson said: "We aim these cameras at houses and those readings are accurate.

"Any objects or areas which are closer to the camera than the house itself may be flagged up as losing heat but that's not what the camera is designed to detect."

With loft and cavity wall insulation, the Government estimates homes could save £160 a year on their energy bills.

The scheme has been tested in cities including Manchester and Brighton.