Bradford assistant manager Colin Cooper has quit the club after accepting a coaching position with Middlesbrough.
Cooper, 44, arrived at Valley Parade in March as number two to Peter Jackson, who unexpectedly resigned two weeks ago.
Phil Parkinson was named as Jackson's successor last week and former Boro and Nottingham Forest defender Cooper accepted his offer to stay on at the club as his assistant, but has since opted for a fresh start.
Parkinson told the club's official website: "It's all happened pretty quickly this week to be honest. He's been made an offer, and it's one that he believes is too good to turn down. We would obviously have liked him to stay, but we have to respect his decision.
"I have to stay that Colin has been excellent with me since I've arrived at the club and I wish him all the best."
Cooper added: "I'd like to thank all the fans for their generous support throughout my time at the club, especially during the week or so that I was in charge after Peter left. I'd also like to wish Phil and the lads all the very best for the future."
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