Officers at Bourne Police Station are unable to return numerous bikes which have been handed into over recent weeks as the owners hve not reported the theft.
Police community support officer Graeme Parrott advised people to report the theft of the bicycle.
He said: "I would encourage people to report the theft and to get their bikes property marked, which they can do by bringing the bike into the police station. This means if it is returned to us we can reunite it with its owner.
"If you see anything suspicious or anyone with a lot of bikes people should report it to Bourne police station or through Crimestoppers anonymously.
"Someone is going around and stealing bikes and we would urge people to be cautious."
During national bike week, which runs from Monday, June 15, the neighbourhood police team will be offering free property marking on bikes.
To report a bike stolen call Bourne police station on 01778 394892.