Thieves are targeting vehicles left at car parks along a 14-mile walk with nine incidents have been reported to police in recent weeks. Patrols are being increased on Brampton Valley Way, the old railway line route, stretching from Boughton Crossing near Northampton to Market Harborough.
The walk is a haven for birds, wildlife and field flowers. It is popular with walkers, ramblers and cyclists.
Patrols began on Saturday and continue until the beginning of September.
Police patrols will be reminding motorists of the need to take everything with them when they park their car to avoid falling victim to thieves.