London - Farmers were served a warning on Thursday after a wild boar broke into a pen at Princess Anne's country estate and killed a prized pig.The attack at Princess Anne's 300-hectare Gatcombe Park property in central England came just a day after another wild boar caused a fatal road accident just a few kilometres away."We had a visit last night on our farm from a wild boar," Anne told a farmers' conference in Oxford. "My Gloucester Old Spots boar is no longer with us as a result."Princess Anne is a keen farmer and patron of the Gloucester Old Spots Pigs Breeders Club, which links lovers of a breed known for its docility and intelligence.Wild boar were hunted to near extinction in Britain but numbers have come back since farming them became fashionable in the 1970s. They escape and become feral.