Thousands of lives are being put at risk every year by unlicensed gas engineers, industry regulators have warned.
It is estimated unqualified fitters were responsible for 1.1 million jobs in 2015, including boiler, cooker and pipework installations.
Gas Safe Register say the illegal work is a serious health and safety concern with people at risk of gas leaks, fires, explosions and carbon monoxide poisoning.
The UK regulating body found more than half of the 5,000 jobs it has investigated in recent years were unsafe, and were forced to immediately cut off the gas supply to one-third of them.
It is now urging people to make sure that their gas fitter is legitimate. All qualified fitters are legally required to be on the Gas Safe Register.
Jonathan Samuel, chief executive of Gas Safe Register, said: “Too many people are letting unregistered gas fitters into their home and putting their families at risk.
“We are urging people to trust the triangle. Look for the Gas Safe Register triangle on your gas engineer’s ID card and make sure you’re getting your gas appliances checked every year.”
The survey found that one in four people found their fitter through an advert, presuming they were legal.
Sixteen per cent said they would take it on trust if an engineer said they were registered rather than asking to see their ID card.
The shocking research also revealed that one in five people did not know that carbon monoxide is invisible, and two in five people were unaware that it has no smell.
Data from the Health and Safety Executive (HSE) states that 22 people have died in the last three years, with a further 1,000 people injured by gas-related incidents.
Gas Safe Register has an up-to-date list of qualified engineers. Call 0800 408 5500 or visit GasSafeRegister.co.uk to find one in your area.
If you suspect a gas leak, call the national gas emergency line which is manned 24 hours a day on 0800 111 999.