CHICAGO — In the first formal push against Detroit emergency manager Kevyn Orr's restructuring plan, Ambac Assurance Corp. Monday called the plan harmful to the city's future and said it has hired an advisory firm.

The public criticism comes in the fourth week of negotiations between the city and its creditors, including the six monoline bond insurers that wrap most of Detroit's bonds. If negotiations break down, Orr has said he could file for Chapter 9 by as soon as the end of the month.

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