Barr, originally signed as a player from Crewe by then Carlisle boss Mervyn Day in 1997, and he quickly established himself as a valuable member of the Brunton Park coaching team after a knee injury forced him into early retirement in 2000. He was also United's caretaker manager for four months after Roddy Collins' first sacking in 2002, Barr becoming one of Simpson's most trusted lieutenants, as well as leading United to a reserve team title last season.
The ex-Halifax man actually applied for the vacant United managerial post when Simpson' replacement was being sought last month, but he was overlooked in favour of Neil McDonald. Barr also turned down an offer from an unnamed League Two club at the start of the season to become their assistant manager, but the lure of Championship football was seemingly too much for him to turn down as his departure leaves United with big coaching staff problems at such a vital pre-season point.