Shares of Countrywide Financial Corp. tumbled more than 17 percent Monday morning after an analyst recommended that Bank of America Corp. abandon its takeover of the troubled lender.

Friedman Billings Ramsey & Co. analyst Paul Miller cut his rating on Countrywide. “Bank of America should completely walk away from the Countrywide deal, as Countrywide’s loan portfolio will prove a drag on earnings and could force Bank of America to raise additional capital,” Miller said.

The report comes after Countrywide’s credit rating was cut to junk by Standard & Poor’s last week due to doubt over whether the bank will cover all of the outstanding debt.

Countrywide has a total of $97.2 billion in debt. The debt includes $11.5 billion in credit lines and $47.7 billion in advances from the Federal Home Loan Bank Board. Bank of America has said it may not guarantee $38.1 billion in other debt.

“There is no assurance that any such debt would be redeemed, assumed or guaranteed,” Bank of America said in a filing last week. The bank added that it had not made a concrete decision on the matter.

Bank of America agreed earlier this year to acquire Calabasas-based Countrywide in a $4 billion stock swap, which is expected to close in the third quarter.

Shares in Countrywide were down to $4.97 in early trading Monday.