The latest figures from the Gas Safe Register have indicated that a possible three quarters of all gas jobs carried out in Scotland homes may be illegal.
The picture isn't much better for the rest of the UK where more than 250,000 gas and heating jobs are carried out by illegal fitters each year.
Keith McKinlay from the Gas Safe Register, the new body which took over from Corgi, is warning home-owners to make sure they check their engineer has the relevant qualifications and is registered by them.
He has said that 75% of the gas installations they have checked in Scotland over the last 10 months have proved to be both illegal and unsafe, requiring immediate remedial action.
He also recommends that home-owners should have their boiler checked by one of their members at least once per year.