It was also found that those most likely to have the keys would be children of the household, as well as other members of family and next-door neighbours.
Discussing the research, Sainsbury's home insurance manager Neil Laird said: "It's concerning to see just how many homeowners have completely lost track of who could have access to their home.
"If you don't take reasonable care of your property, including keys, you could potentially invalidate your insurance."
Sainsbury's Bank recently warned cyclists to ensure they take steps to ensure their bikes are safe at home.
The provider made the comment after Conservative leader David Cameron had his bicycle stolen.
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