Jack, 28, follows Crusaders team-mates Aaron Mauger and All Blacks colleagues Carl Hayman and Byron Kelleher in moving offshore.
The 59-Test second-rower said in a statement: "I'm just very thankful for the opportunities that I've had, both playing alongside some very special men, but also in some fantastic teams.
"I'd like to thank all of the coaches and players who have helped me along the way.
"Finishing after the Rugby World Cup and heading over to England will give us the opportunity to experience something different, both in a rugby sense, but also in lifestyle."
Crusaders coach Robbie Deans said: "You just have to look at the volume of games that he's played, and the performances he has been involved in, to see what a significant player Chris is to the Crusaders.
"His record speaks for itself. He is going to be a huge loss, both for the Crusaders and for New Zealand rugby in general, but the quality and length of his service has been such that no one could begrudge he and (wife) Kym their opportunity to head off and see the world."
Jack will join New Zealand Maori fly-half Glen Jackson at Vicarage Road.