Watch out for the zombies.

The plethora of companies propped up by the European Central Bank will limit policymakers' ability to withdraw monetary stimulus that has been supporting the continent's bond market since the financial crisis, according to strategists at Bank of America. About 9 percent of Europe's biggest companies could be classified as the walking dead, companies that risk collapse if the support dries up, according to the analysts.

After the crash of Lehman Brothers sent global markets into a tailspin, a decade of easy-money policies gave breathing room for nations to get their balance sheets in check and allowed for a spirited revival in corporate profits. But as central bankers look to pull back stimulus for fear of overheating, the potentially grim outlook for vulnerable companies may give them pause, according to Bank of America.

"Monetary support in Europe over the last five years has allowed companies with weak profitability to continue to refinance their debt and stave off defaults," analysts led by Barnaby Martin wrote in a note Monday. "This supports the point that our economists have been making: that the ECB will likely be very slow and patient in removing their extraordinary stimulus over the next year and a half."

The strategists classify zombies as nonfinancial companies in the Euro Stoxx 600 with interest-coverage ratios — earnings relative to interest expenses — at 1 or less. The thinking goes that companies in this category are particularly vulnerable to rising interest rates.

About 6 percent of European companies had a coverage ratio of less than 1 on the eve of Lehman's downfall, a percentage that fell to as low as 5 percent in 2013 when the euro-area sovereign debt crisis cooled. Zombies shot up to as high as 11 percent in June 2016 before easing in recent months.

Energy companies and those based in southern Europe make up a disproportionate share of the zombie world, according to Bank of America.

To be sure, different metrics tell different stories about the health of corporate leverage, with some investors citing growth projections and yardsticks like net debt to earnings as reasons bond buyers can be more sanguine. But the ECB risks a big backlash if it were to tighten policy prematurely.

Allowing a "credit tantrum to take hold would only pressure corporate interest costs again, and risk a rise of the zombie," Bank of America strategists conclude.

Sid Verma writes for Bloomberg.