Bill Ackman, the hedge fund manager, has amassed 5m shares in Sears during the past month, a sign that patience with the department store operator’s turnround efforts may be finite.

For now, Mr Ackman’s 3.5 per cent stake, at a cost of about $675m, is meant to prod Eddie Lampert, the chairman, along in his mission to boost Sears’ value. But, if history is any guide, Mr Ackman will have some suggestions of his own.