And it’s raised the possibility of a return for Tony Harnell, who fronted them for 22 years before bowing out in 2006.
Mills doesn’t think his decision will come as a surprise, following the death of keyboardist Dag Stokke in 2011 and Victor Borge’s decision to move on last year.
He says in a statement: “After the passing of our beloved Dag and the parting with Victor, my departing may not be altogether unexpected.
“There are other contracts I need to fulfil and a different focus I need to maintain in the coming years. I wish Ronni Le Tekro and his team all the best for their future, however it may shape itself.
“We’ve had some remarkable experiences together and we’ve worked with some incredible characters.”
Mills has vowed to remain until the band’s current commitments have been fulfilled, including two shows in Norway this weekend.
Meanwhile, Harnell has refused to go into detail about rumours suggesting he’ll return to TNT. “The ‘press leak’ in Norway was a bit premature,” he says. “Meaning: if anything were to happen it’s not happening this soon – and I’m not saying anything is going to happen.
“The beginning and the end are sometimes a little too close to each other. But the door of possibility always remains open.”
Guitarist Le Tekro is the only founding member remaining, and the only person to have appeared in all lineups of TNT.