A gas fitter caused the deaths of his partner and her 10-year-old daughter by using a generator to heat their boat while carbon monoxide sensors were disabled, prosecutors claim.
Matthew Eteson is on trial at Preston Crown Court for the manslaughter by gross negligence of Kelly Webster, 36, and Lauren Thornton.
The pair died in their sleep aboard a boat on Windermere in April 2013.
Mr Eteson, 42, from Hale in Greater Manchester, denies the charge.
The family had gone to the Lake District for the Easter bank holiday weekend to celebrate a friend's birthday.
But on the afternoon of 1 April, Mr Eteson removed the generator from the deck and used it to power a 1kW fan heater to heat the sleeping quarter, prosecutor Mr Graham Reeds QC said.
The court was told that there had been no warning to the build-up of the toxic gas because the carbon monoxide sensors had previously been disabled.
Mr Reeds said the mother and daughter, both from Leyland in Lancashire, were found dead in their sleeping positions aboard the the second-hand Bayliner 285 motor cruiser Arniston, with fatal levels of carbon monoxide in their bodies.
Mr Eteson, who had been sleeping in a separate compartment further away from the generator also succumbed to the effects of carbon monoxide poisoning but did awake to find the bodies.
The court also heard Mr Eteson, a qualified gas fitter, had made some amendments to the generator and its exhaust, which experts subsequently deemed to be "dangerous".
Ms Webster's mother, Nia Webster, had previously asked Mr Eteson about carbon monoxide sensors.
He told her they were not working and his partner would unplug a portable detector whenever she was cooking.
The trial continues.