Kent Trading Standards said there had been at least two incidents in the Thanet area when gas boilers had to be turned off because of the danger.

The roofers are turning up on doorsteps asking for work and are targeting elderly householders.

Chimneys were left unable to act as a safe escape route for boiler fumes.

Nobody hurt

Mark Rolfe, East Kent area manager for the county's trading standards, said: "These are people who are claiming to be roofing contractors.

"They do seem to do work on the roof, but they don't seem to know what they're doing and they're actually putting people's safety at risk.

"In the course of the work they've lowered some of the chimney stacks on these houses... and rendered the gas boilers that were using the chimneys as their exhaust routes unsafe.

"At the moment we've managed to catch this to a point where nobody's been hurt," he added.

"But my great concern is that there are others where this work has been carried out that we don't know about."