Campaigners who have removed "welcome to Lancashire" signs will be reported to police, the county council says.
More than 30 signs were targeted by the group
The CountyWatch group has taken down more than 30 of the signs across Lancashire because it wants historic county boundaries restored.
It plans to put them where it believes the outskirts of Lancashire are.
Lancashire County Council told BBC News it would be speaking to police about the group, which has targeted other English counties.
It said the cost of replacing the signs would be met from taxpayers' money.
Tony Bennett, CountyWatch campaigner, said: "The sign says 'Welcome to Lancashire', it doesn't say 'Welcome to the administrative unit of Lancashire County Council - that would be all right."
The county boundaries marked by the signs were created as part of the Local Government Act in 1972.
Those borders targeted by CountyWatch include the boundaries between Lancashire and Greater Manchester and Merseyside.
Lancashire County Councillor Tony Martin said: "We are the only local authority that promotes the county on its signs so it is a shame that people are spending time, and wasting taxpayers' money, in this way.
"Our name is Lancashire County Council - what else are we supposed to put on signs?"