Alex Mitchell was found dead at a house in Cwmbran

Cardiff Crown Court heard how Scott Lee Stuart, 38, from Newport carried out the work at Alex Mitchell's family home in Cwmbran in March 2005.

Two days later she and her pet dog were found dead from carbon monoxide poisoning.

He also admitted two charges contrary to health and safety regulations.

The court heard 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.

'Talented daughter'

Stuart was registered to fit central heating boilers but not flueless gas fires.

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.

Stuart will be sentenced in October.

A family spokesman said at the time of her death: "Alex was a wonderful, most beautiful and talented daughter and grandaughter."