Rectifying botched illegal gas jobs is costing customers £100 million a year, the Gas Safe Register has said.
The register, which replaced the Corgi registration scheme in April 2009, said illegal gas fitters were not just costing money but also putting people's lives and homes at risk.
It claimed 250,000 illegal gas jobs were carried out every year, with many carried out by fitters recommended by family and friends but with no official qualifications.
Phill Brewster, national investigations manager at the Gas Safe Register, said: "If you have a boiler fitted by a gas engineer who isn't registered to do the work, then you may end up paying to have it ripped out and start again which may cost you thousands of pounds. It may also put your life in danger."
Research carried out by the register revealed that one in three people would trust a tradesman on recommendation alone.
Mr Brewster urged people to check fitters were registered to work legally and have the correct qualifications.
He added: "Money may be tight so it's understandable that people ask around for recommendations in the hope of getting a good job done.
"But with 250,000 illegal gas jobs done every year by cowboys, it's shocking that five times as many people would hire a gas engineer simply on trust rather than checking they're safe, legal and Gas Safe registered."
Gas fitters are legally required to be on the register, which can be accessed at www.gassaferegister.co.uk.
