Chairman Peter Blackburn, who will assume the role of executive chairman until a replacement is found, said Stewart had left by "mutual agreement" as the company had consistently failed to operate to its full potential.
He said, "We have actually performed well from a sales point of view - the problem has been translating that into profit. There is no doubt we have underdelivered in recent times."
Grocery profits are expected to improve on the previous year, despite the well-publicised problems in the biscuits division, said Blackburn.
However, the convenience foods division had been hit by the loss of business to competitors, the hot summer and soaring raw material costs that the company had not been able to pass on through the supply chain.
Second half pre-tax profit is expected to be "around the level achieved in the second half of last year".