Jansen, who was chief operating officer of Sodexo and CEO of its European business, was due to start at Brakes on 1 February, although he was to temporarily retain positions on Sodexo's German board and with an MoD project in Wales involving Sodexo.
A spokeswoman for Jansen's current employer, Sodexo, said it had taken "longer than expected to wind things up at Sodexo".
But one industry source claimed that Jansen's appointment had been delayed because Compass Group, which is Brakes' biggest customer and a great rival of Sodexo, had complained about Jansen's continued involvement with Sodexo.
"Jansen is involved with a couple of major projects that he couldn't get out of straight away," said the source. "Brakes' biggest customer was not happy with this arrangement. But it seems Brakes is in good hands in the meantime, being run by a really competent team."
Brakes' chief operating officer Ian Goldsmith is in temporary charge of the business, following the retirement of Frank McKay, although McKay remains "on-call for if needed".
Compass whose multimillion pound contract with Brakes is due for renewal in the next two years admitted that it had complained about the dual role that Jansen was due to take but denied this was the cause of the delay.
McKay admitted the 1 February start date had been "wishful thinking". "That Jansen is not in post already is not having an impact on the company whatsoever," he added.
Brakes said in a statement: "Jansen's confirmed start date is 1 July 2010 by which point he will have discharged all his remaining obligations with Sodexo. Both Brakes and Sodexo are very happy with these arrangements."
A Sodexo statement added: "Jansen will now be leaving Sodexo at the end of June and Brakes is happy with this arrangement."