Over 6,000 robberies have taken place last year in the UK because of the common practise of poorly hiding the spare keys under the mat, according to a new study.
The keys were either stolen at an earlier date or found in a hiding place near the premises, such as under a plant pot or doormat.
"Almost a third of Britons (29 per cent) admit leaving a 'spare' hidden on their property, despite the security risks," it said.
Those leaving a key out believe it is safe to do because they leave it unattended for short periods of time or think they have a "really good hiding place", while others believe it is okay to do so because they live in a safe neighbourhood.
Main reason for leaving a key out is so that friends and family can get into the house while they are away from home.
"More than 6,000 burglaries in 2014 involved intruders using keys to gain access to the property, as opposed to breaking and entering," official police figures said.
The most common places to hide a key are under a plant pot (19 per cent), bin (13 per cent), rock or stone (13 per cent), doormat (10 per cent) or a gnome or garden ornament (eight per cent), according to a survey of 2,000 adults.
The research found that, on average, each household has lost at least two keys equating to nearly 20 million spare keys currently unaccounted for.
"While the number of burglaries is falling overall, it is alarming to see that the number of thefts where the burglar used a key is increasing," said Selwyn Fernandes, convener of the survey.
"Burglars know that people tend to leave a spare key in a handful of places near their door and will often search these before attempting a break-in," he said.
