Clarke joins from Kraft Foods, where he was president of its European operations, on 1 September, replacing outgoing CEO Robert Schofield.
City experts said an external candidate was the only option for a company battling against unprecedented input costs.
"Making an internal appointment would have gone down very badly," said one source. "Generally the City prefers a bit of distance between chairman and CEO but there's a connection with Ronnie Bell who was also at Kraft."
Clarke would understand how to bargain with the mults, added another source close to Premier.