MADISON, Wis. -

A group of state prison guards has filed a lawsuit, alleging the state owes them millions in back pay because of a 2012 work rule that defines when they are considered on duty.

A group of 10 prison guards from institutions across Wisconsin filed the lawsuit Tuesday in Dane County Circuit Court.

According to the lawsuit, the state Department of Corrections issued a policy on Jan. 29, 2012, stating that employees are considered "on duty when they are present at their assigned post/work location prepared to assume their duties at their designated start time."

But the guards allege it means they aren't paid for pre-shift and post-shift work that serves DOC's interests.

State Department of Corrections spokeswoman Joy Staab said she couldn't comment Thursday because the lawsuit is ongoing.