Gas Safe Register estimates that there at least 7,500 illegal gas fitters working on jobs without the skills or the qualifications to work on gas.
The body said that more than half of these operators are sole traders.
It said that, as well as carrying out gas work, up to 90 per cent of these illegal fitters also undertake plumbing jobs and 81 per cent carry out bathroom fitting while two thirds also do kitchen fitting and over 40 per cent carry out general building (43 per cent) and electrical work (43 per cent).
Gas Safe Register’s national investigations manager Phill Brewster said: “Gas and cowboys do not mix, it’s a fact. One in 10 of the illegal gas installations we investigated in the last nine months have been Immediately Dangerous and we had to disable the appliances straight away to make them safe.
“Almost half of these had faults that could lead to lethal carbon monoxide exposure. So, with our research showing that illegal gas fitters do 250,000 jobs in our homes each year, it’s worrying to think how many gas appliances out there have been left in a dangerous state.”
Mr Brewster stressed that a qualification to work on one gas appliance, for example gas boilers, does not mean a fitter is automatically qualified to put in a gas fire.
He added: “If they do this, the work is illegal and potentially life threatening. The work that engineers can legally do is listed on the back of their Gas Safe Register card – it’s so important you check this every time you have work done.”