A prank by a US prison inmate has been uncovered more than three years after the joker changed a police car decal design to include a hidden image of a pig.

A Vermont State Police officer discovered the cheeky porker on a patrol car late last month.

It is believed the decal has been applied to approximately 30 Vermont State Police cruisers since 2008, when a prisoner working in the Vermont Correctional Industries Print Shop changed a spot on a cow on the police logo into a pig.

Police say the prankster, who was responsible for designing and printing the decals, must have modified the computer program that contained the traditional state emblem.

Police have launched an investigation into the prank, although they admit the inmate responsible may no longer be in custody.

If he is identified (potentially by another inmate squealing on him), the offender is likely to be disciplined internally rather than face any criminal charges.

Vermont State Police says it will cost around US$780 ($725) to replace the modified decals.

Vermont public safety commissioner Keith Flynn told the Burlington Free Press he could see the funny side, and admitted police did not find the term ‘pig’ as offensive as they did in the past.

“If the people had used some of that creativeness, he or she would not have ended up inside,” Flynn said.

Vermont State Police executive officer William Sheets said the quality control at the Corrections Department would be improved in the future.

Note: Image courtesy of Burlington Free Press.