Stuart had admitted manslaughter at an earlier hearing

Scott Lee Stuart, 38, from Llanmartin, near Newport, south Wales, was not qualified to carry out the work at Alex Mitchell's family home in Cwmbran.

Two days later the teenager and the family's terrier were found dead from carbon monoxide poisoning.

Stuart admitted manslaughter at a hearing last month.

At that hearing Stuart also admitted two charges contrary to health and safety regulations.

Those tradesmen that purport to be qualified and are not will be held accountable under law where injury and death occur Det Chief Supt Ray Wise, head of Gwent CID

The Health and Safety Executive (HSE) said after the case that although Stuart was "registered with Corgi (Confederation for the Registration of Gas Installers) to fit boilers, he had not had his competence to fit gas fires assessed by a recognised body".

HSE inspector Hugh Emment, who helped the police investigation, said: "This tragic case proves the dangers posed by faulty or incorrectly fitted or tested gas appliances and the potential for CO (carbon monoxide) poisoning.

"I would urge members of the public having gas work carried out to ensure Corgi-registered personnel do it."

He said it was a legal requirement for gas fitters to be Corgi-registered, which must cover the type of appliance being fitted, and urged anyone having work done to check both.

The court heard then how Stuart, who ran Stuart Plumbing and Heating Services, failed to check that the gas pressure on the appliance was set at the correct level, which led to a leak.

Alex's mother found her dead on her return from work

He was registered to fit central heating boilers but not have the relevant training to fit flueless gas fires, the court heard.

On the day Alex died, she had been at home alone with the family pet dog.

When her mother Ann returned home from work she found the terrier dead in the hall.

She then ran upstairs where she found her daughter's body in her bedroom.

Despite efforts by neighbours, Alex could not be revived.

At the time a family spokesman paid tribute to the teenager saying: "Alex was a wonderful, most beautiful and talented daughter and granddaughter."

Det Chief Supt Ray Wise, head of Gwent CID said the case "sends out a clear message that those tradesmen that purport to be qualified and are not will be held accountable under law where injury and death occur".

"They are playing with people's lives and the public must be alert to the fact that such people are operating in our community," he added.

About 20 people die each year from carbon monoxide fumes at home, and the HSE said two-thirds of these could probably have been avoided with servicing by a competent installer.