Associated Industries of Massachusetts has joined Procter & Gamble, 3M and other companies in challenging a proposed insurance-company acquisition that could leave scores of Bay State businesses exposed to so-called long-tail asbestos and environmental liabilities.

The companies and AIM last week filed a petition to intervene in the Pennsylvania Insurance Department review of Trebuchet US Holdings Inc.’s proposed $2.2 billion purchase of OneBeacon Insurance Co. and Potomac Insurance Co.  Trebuchet, a subsidiary of the Bermuda-based Armour Group Holdings Ltd., filed its application to acquire the two companies in February.

AIM and the other petitioners fear the proposed transaction would impede the ability of thousands of companies to get claims paid under environmental insurance coverage purchased from a predecessor to One Beacon, the Commercial Union Insurance Companies.

“Under the proposed transaction, those legacy policies, and the claims-paying obligations that are part of their core promise, will be jettisoned by (One Beacon) and shunted over to a runoff operation with a suspect capital structure and limited resources for satisfying valid claims under hundreds, if not thousands, of legacy policies issued by predecessor entities��_” the petition said.

The document indicates that AIM and the other petitioners “are concerned that the conversion of the Acquired Companies into a runoff vehicle, operating with a significantly reduced policyholder surplus and decoupled from (One Beacon’s) ongoing business operations and pooled reserve and reinsurance structure, may impair the financial viability of the Acquired Companies and compromise their ability to pay valid claims arising under the legacy Commercial Union policies.”

The policies that the companies bought were in effect from the 1930s until the 1980s and provided “broad protection” against claims and lawsuits brought by third parties seeking compensation for personal injuries or property damage that allegedly occurred during the relevant policy period. The policyholders were covered even if the lawsuits or claims filed against them occurred many years after the alleged acts were committed and even if the injuries or damage weren’t apparent during the coverage periods.

The companies seeking to intervene with Pennsylvania authorities say they had received “substantial numbers” of asbestos, environmental, toxic tort and other claims seeking financial recovery for “latent bodily injury and property damage that allegedly developed over the course of many years.” The list of petitioners includes the Pennsylvania Manufacturers Association, ITT Corporation, Pentair Ltd. of Mansfield, Invensys of Foxboro, Belden Inc. and The William Powell Company.

Robert Rio, Senior Vice President of Government Affairs at AIM, said the association cannot estimate the number of Massachusetts manufacturing companies with liability policies that could be affected by the OneBeacon acquisition. He noted, however, that Commercial Union was a large writer of property/casualty policies in Massachusetts for decades.

“The broad coverage provided by these legacy policies is both valuable and irreplaceable, as policies providing comparable protection are no longer available at any price in the current insurance markets,” Rio said.

The petition to intervene in the case also raises concerns about the fact that most of the exhibits submitted by Trebuchet – including financial statements, business plans for the acquired runoff entities, and reinsurance arrangements ” are classified as “confidential,” “proprietary” and “trade secret.”

The companies maintain that the “pervasive non-disclosure of these materials appears designed to render the Proposed Transaction as opaque as possible, and to deprive policyholders whose rights may be impaired by the Proposed Transaction of any meaningful opportunity to assess its financial impact on the security of their policies.”