Gas fitters have been warned that they face fines and injury compensation claims if they do not have the proper credentials.
The warning follows the prosecution of Peter Tongue, a gas fitter from Brecon, who was jailed after two people died from carbon monoxide poisoning from a central heating system he had worked on.
Mr Tongue was found guilty of manslaughter in each case and sentenced to three years in prison for each death, after it emerged that his accreditation to work on that particular system has lapsed three years previously.
Health and Safety Executive inspector Hugh Emment commented: "This sends out a stark message to gas engineers that working outside their assessed competence can costs them their liberty but more importantly can cost lives.
"This also serves as a warning to those who are doing gas work who are not on the Gas Safe Register."
Last month companies were reminded to keep their gas appliances in check after the HSE launched an initiative to visit and check catering businesses around Warwickshire.