Insurance company rating agency A.M. Best Co. placed its grading on Manulife under review for a possible downgrade following the company's $6.4 billion hostile takeover bid for Canada Life.

A.M. Best currently has an A++ (Superior) rating its highest grading of financial strength on Manulife and several of its U.S. subsidiaries. The rating company has an A+(Superior) grade on Canada Life.

"While A.M. Best views this offer as a key component of Manulife's growth strategy to become a leading global financial services company, this transaction involves a firm with relatively weaker ratings and possible integration challenges inherent in an acquisition of a firm that has international businesses in markets which Manulife does not operate," the rating company said in a statement.

"Moreover, as this offer is an unsolicited one, its structure may complicate integration activities given the limited due diligence performed on Canada Life," A.M. Best said.

Manulife's rating will stay under review while A.M. Best conducts discussions with the company's management on plans to integrate with Canada Life.