People who move house are being urged to change the locks, as failure to do so exposes them to increased risk of burglary.
The findings comes from new research commissioned by Direct Line and show that 63% of people moving into a new home do not bother to change the locks.
More than 70% of people have a spare key looked after by a family member, with 22% handing on to neighbours.
Direct Line’s research also finds a number of basic home security errors are common, particularly leaving a spare key outside (often under the doormat).
More than 3.4m people (7%) have a spare key outside, and these can prove greatly helpful to burglars, whether hidden under a doormat, or plant pot, or elsewhere.
Direct Line’s Matt Owen described the failure to change locks when moving into a new home and keeping a key outside as handing a gold-plated invitation to burglars.
Last week Direct Line warned homeowners of widespread underinsurance, with £212.9bn in home contents not covered in the UK.