Police are warning West Country construction workers to be on the alert for thieves targeting vans and stealing either the vehicles or tools.
Avon & Somerset Police say the thieves, often themselves in a white Ford Transit van, are targeting vehicles at building sites or parked at builders' merchants and large hardware stores.
The thieves have been described as white men, sometimes wearing fluorescent tabards. They are able to break into a van and make off with tools within seconds.
There have been more than 30 reported incidents in the past month in the Bristol and South Gloucestershire areas, most between 8am and 2pm.
South Gloucestershire community safety inspector Gerry Treacy said: "I'd ask people to call us immediately if they spot anyone behaving suspiciously around vans, if they have been offered tools for sale in suspicious circumstances or if they any other information which could help our enquiries.
"Anyone with information can contact police on 0845 456 7000. Alternatively contact the independent charity Crimestoppers on 0800 555 111 or through www.crimestoppers-uk.org. They never ask your name or trace your call.
"It's also really important for people to permanently mark their property with their business name or postcode, and again with an ultraviolet pen. This will deter thieves because it will enable police to identify items as stolen."