A leading supplier of replacement doors and windows to the insurance industry is calling for glazing contractors and operators to be licensed.

At least two thirds are "cowboys" and their poor workmanship could result in a fatality unless action is taken, according to Phil Martindale, founder and managing director of Bolton-based Martindales.

Some new businesses setting up do not know that the glazing equivalent of the MOT, "Document L", even exists, he says.

"Doors and windows are being fitted incorrectly by these cowboys, and in some cases dangerously under-specified glass is being fitted in doors and windows in homes where toddlers are charging around.

"The consequences hardly bear thinking about."

The cowboys generally fall into two categories, says Mr Martindale, whose ten branches include one in Wolverhampton.

"Firstly, the fitter whose unlettered van looks like a skip, with no proper racks.

"He's a mess, covered in bling and mastic, no over-alls, poorly prepared with no dust sheets - and certainly offers no sort of customer service.

"Then there's the second category - good people who are skilful with their hands, who are largely honest but blissfully ignorant of the regulations relating to the installation of replacement windows or doors.

"The first category has cowboy written all over, the second is potentially the silent killer. The best they can provide is a window or door that might do the insulation job it was supposed to.

"But the worst could be anything from a huge devaluation of the house because more and more house sales are dependent upon the right documentation, through to the installation of an incorrectly specified piece of glass that somebody could too easily break or even fall through."

Building regulations state that windows must comply with standards laid out in a joint government and industry self-assessment scheme called the Fenestration Self-Assessment Scheme, or FENSA.

"Some installers are affiliated to FENSA and provide installation to the required standard and achieve Document L standard, but many do not. Those who do not often take advantage, because installations by non-FENSA members should be checked and assessed by the local authority but it's down to the householder. It's human nature - many do not. This leaves the door wide open for anybody to wake up one morning and set up a replacement window installation business.

"A few hundred quid on a van, credit from a supplier, and they're off - and we know there are lads fitting the thick end of £30,000 worth of unregulated replacement windows a month."