There were "serious failings" in the way home appliance firm Beko acted when it found its gas cookers had the potential to emit fatal levels of carbon monoxide, a coroner has found.
An inquest heard five people in Cornwall died after inadvertently turning on their grills.
Coroner Geraint Williams concluded they died as the result of an accident.
The cookers have been linked to 13 other deaths in the UK and Ireland, the inquest heard.
Kevin Branton, 32, and Richard Smith, 30, died in 2010 in Saltash, while Maureen Cook, 47, Audrey Cook, 86, and Alfred "John" Cook, 90, died in 2013 in Camborne.
Cornwall Coroner's Court heard that if the grill was used with the door shut, fatal levels of the poisonous gas built up due to a design fault with a rubber seal around the door.
Summing up the six-day inquest, Mr Williams said it was "glaringly obvious" a grill might be deliberately or accidentally used with the door closed and this issue should have been recognised by Arcelik, Beko's parent company, which manufactured the cookers.
"This singular failure led ultimately to the deaths of Mr Smith, Mr Branton and the Cook family," he said.
The inquest heard Beko became aware of the first fatality - that of French student Alexis Landry in Ireland - on 13 November 2008.
It was contacted about two further deaths in Doncaster on 1 December.
Mr Williams said Beko's "failure to pursue" more information about the Doncaster deaths was a "serious error".
He found that a delay in Beko sharing information about testing results and further deaths was a "serious failing" and said there was a "lost opportunity" to stop Mr Smith's father from buying his cooker on 31 December - or to obtain his details from the retailer Co-op Homemaker in Plymouth, before it went out of business in 2009.
In a statement released after the hearing, Mr Branton's mother Denise said she believed "Beko should and could have reacted quicker and more proactively".
"If they had, I'm sure the cooker that killed our sons would not have been able to be purchased," she said.
"It would appear that there is a likelihood that some of these cookers may still be in use in some homes today.
"I strongly urge people who know of someone who owns an older cooker to check Beko's list of recalled cookers online as they are not all sold under the Beko name."
Mr Smith's father Brian said the cookers, which were not tested with their grill doors closed, should have undergone "every conceivable test" before being sold.
He said: "I hope that the lessons learnt during this inquest are taken on board by all persons concerned who have a duty of care for public safety and regulations are changed to prevent further deaths."
Mr Williams said he would now consider whether to make any recommendations to prevent future deaths.
In a statement Beko said its main objective was to "ensure that every Beko product is safe for our customers".
A spokeswoman said: "Since these tragic incidents, we've continued to raise our safety standards... We also collaborated with the industry to get the UK and EU gas safety standards changed in 2009.
"The new standard helps prevent a similar tragic event from happening again."