Following a number of incidents, including some in Sheffield, South Yorkshire Police is warning delivery drivers to be on their guard.
The force said thieves are prowling the streets looking for parcels to steal as well as vehicles.
Detective Inspector Ben Wood said: “We are seeing an increase in delivery drivers being targeted for theft.
“The reports we are receiving have a similar trend of drivers leaving their vehicles unlocked and thieves are taking advantage of this opportunity.
“Alongside this being distressing for the delivery driver and having an impact on our communities, stolen cars are more likely to be used in further crimes, which creates additional risks for our communities.
“We can all take simple steps to avoid this happening.”
He added: “To keep yourself, belongings and vehicle safe, please follow these simple steps:
“Lock your car every time you get out of it, including as you deliver a parcel to a door, although it is a matter of minutes, thieves will take advantage.
“Do not advertise your personal belongings, keep mobile phones, money and satnavs out of view.
“If possible keep parcels out of view.
“Do not ‘advertise’ the fact that you use your personal vehicle to deliver parcels.
“Trust your instincts, if you think someone is watching you or following you, call 101 and report it.”
To pass on information about those targeting delivery drivers call 101 or Crimestoppers, anonymously, on 0800 555111.