Gas fitter Matthew Eteson, 42, has been found guilty of the manslaughter of mum and daughter Kelly Webster, 36, and Lauren Thornton, 10.
The pair from Leyland were overcome by carbon monoxide fumes from a faulty generator Eteson had brought below deck on his boat, moored on Lake Windermere.
Kelly’s partner Eteson, of Appleton Road, Hale, denied gross negligence manslaughter at a trial at Preston Crown Court.
However on Monday, the jury unanimously found he had caused the deaths of Kelly and Lauren by carrying out unsuitable and poor quality work to the generator.
As the guilty verdicts were returned, members of the Webster and Thornton families sobbed and comforted each other in the public gallery. Eteson and his father looked down.
Sentence was adjourned until November 13.